French Onion Mushroom Casserole Recipe

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French Onion Mushroom Casserole in cream colored baking dish
French Onion Mushroom Casserole Recipe
Prep Time
20 mins
Cook Time
1 hr
Total Time
1 hr 20 mins
 
Imagine the best bowl of French Onion Soup you've ever had, only in casserole form.
*Peeling pearl onions can be very easy if you follow this step-by-step photographic guide.
Course: Main Course
Cuisine: American, French
Servings: 6
Calories: 458 kcal
Author: Valentina K. Wein
Ingredients
  • olive oil for the pan
  • 1 3/4 pounds pearl onions (click here for How to Peel Pearl Onions)
  • 8- ounces Crimini mushrooms, washed and dried, thinly sliced
  • 1 1/2 tablespoons thyme, finely chopped
  • 2 cups beef broth
  • 1/2 cup dry red wine
  • 5 tablespoons unsalted butter, softened, divided
  • 2 tablespoons all-purpose flour
  • 1 2 foot, skinny French baguette
  • 1 1/2 tablespoons garlic, minced, divided
  • 1 1/2 cups Swiss cheese grated, divided
  • sea salt and freshly ground black pepper
Method

  1. Preheat the oven to 375 degrees F, and adjust a rack to the top third.
  2. Once you've peeled the onions (here's how) -- coat the bottom of a large sauté pan with olive oil and place it over medium heat.
  3. Add the peeled onions and sauté them, stirring periodically, until they're nicely caramelized, 15 to 20 minutes.
  4. Add a bit more oil to the pan, and add the mushrooms. Again, stirring periodically, sauté until they're nicely caramelized, about 10 minutes. Mix in the thyme and cook until it's very aromatic, about 30 seconds.
  5. Turn he heat to low and pour in the beef broth and wine to deglaze the pan. Use a wooden spatula (if you have one -- otherwise a spoon), to gently scrape any bits of stuck onion and/or mushroom from the bottom of the pan, and incorporate it into the liquid. Turn the heat to high and bring to a boil. Then reduce the heat to low again and simmer.
  6. Meanwhile, add 2 tablespoons of the butter to a small sauté pan and melt it. With the heat on low, whisk in the flour. (This is a roux and will thicken the sauce.)
  7. Using a whisk, mix the roux into the onions. Again, turn the heat to high and bring to a boil. Then immediately reduce the heat to the lowest setting and let this simmer, uncovered, for about 15 minutes. Season to taste with salt and pepper. (Here's How to Season to Taste.)
  8. While the sauce is simmering, cut about about 3/4 of the baguette into thin slices, diagonally -- about 1/2-inch thick. Place the slices on a baking sheet and set aside.
  9. In a small bowl, mix 2 tablespoons of the butter with 1 tablespoon of the garlic and spread each slice of bread with it. Sprinkle with salt and pepper and place the baking sheet in the preheated 375 degree F oven until the edges are golden, 10 to 15 minutes. Remove from the oven and add the now toasted slices to the bottom of an (approximately) 9 x 13 x 2-inch baking dish. Set aside, and leave the oven on.
  10. Cut the remaining 1/4 of the baguette into small cubes and add them to the same baking sheet you used for the slices. Mix the remaining 1 tablespoon of butter with the remaining 1/2 tablespoon of garlic and melt it in the microwave, or a tiny sauté pan. Drizzle this over the cubed bread on the baking sheet and sprinkle with salt and pepper. Toss gently with your hands to be sure it's well mixed. (Be careful as the baking sheet might still be hot!) Place this in the oven (still at 375 degrees F), until golden, 10 to 15 minutes. Remove from the oven and set aside. Leave the oven on.
  11. Sprinkle 1/2 cup of the cheese evenly over the toasted slices of bread in the baking dish.
  12. Pour the onion sauce evenly on top and then evenly distribute the cubed, toasted bread on top of that.
  13. Sprinkle evenly with the remaining 1 cup of cheese.
  14. Bake in the (still) preheated 375 degree F oven until it's bubbling along the edges, about 20 minutes.
  15. Let it cool for about 10 minutes and then serve.The “creative and comforting” part is not only about the end result, but is so much about the cooking process.  While I always loved cooking, it wasn’t until I studied it in Culinary school, that the process became more important to me. I realized that creating the best comfort foods was more than just putting ingredients together — it’s all about how they’re put together.
    French Onion Mushroom Casserole Recipe sliced on a white plate with green thyme sprig and red dish in backgroundLearning how and when to add certain ingredients to a pan, what to finish with and why — it all just fascinated me.
    Patience is imperative.
    You must not deglaze the pan until you’ve properly browned the ingredients.  pearl onions in skillet for French Onion Mushroom Casserole Recipe You must wait several seconds after adding herbs, before moving on.
    Let the aromatics come out into the room first.pearl onions in skillet for French Onion Mushroom Casserole Recipe You must let a sauce thicken properly.
    If you add a thickener, but don’t let it bubble, you will not see what consistency you’ve reached.  pearl onions in brown sauce for French Onion Mushroom Casserole RecipeLayer ingredients carefully.
    The order in which foods are layered can make a difference in how each bite feels in your mouth. toasted slices of baguette with white grated cheese on top for French Onion Mushroom Casserole RecipeYou must not pull things from the oven too quickly — it’s not about time, it about the aroma and the appearance.
    Get it?
    toasted croutons for French Onion Mushroom Casserole RecipeIt’s been years since Culinary school, but I still find so much pleasure in the process.  Sure, a quick and easy dinner is essential on some nights, but when time allows, I could spend all day in the process. That’s part of how I achieve the best comfort foods.
    The entire process begins with an idea, of course.  In this case, the idea was born when Melissa’s Produce delivered a huge bundle of gorgeous multi-colored pearl onions to my door.
    What was I to do?  Make them the star of a dish, obviously.
    pearl onions for French Onion Mushroom Casserole RecipeI absolutely love this French Onion Mushroom Casserole Recipe and I think you’ll agree that it’s one of the best comfort foods of all time.