French Onion and Mushroom Pasta Bake

Ingredients

Pasta Bake:

• Olive oil 
• 2 yellow onions, peeled and sliced
• Kosher salt 
• 8 ounces white button mushrooms or cremini mushrooms, sliced 
• 6 tablespoons of unsalted butter
• 1 teaspoon fresh thyme leaves (from about 3 sprigs) 
• 6 tablespoons all-purpose flour
• 1 1/2 cups beef broth (or water) 
• 1 1/2 cups milk  
• 1/2 teaspoon crushed red pepper 
• 1 teaspoon Dijon mustard
• Freshly ground pepper
• 1 cup grated Gruyere + 1/2 cup grated Gruyere (for the topping) 
• 1 cup grated mozzarella (about 5 ounces) + 1/2 cup grated mozzarella (for the topping) 
• 1 pound of pasta of choice (I used rigatoni) 

Bread Topping:

• 2 tablespoons unsalted butter
• 2 cups cubed bread

Instructions

1. 1
   To a medium pot, set over medium-low heat, add two tablespoons of olive oil. When warm, add the sliced onions and a few generous pinches of salt. Mix until the onions are evenly coated with olive oil and cover the pan for about 5 to 10 minutes, mixing every so often to make sure they’re not burning. Cook for an additional 20 minutes, mixing every so often until they’re caramelized, a medium golden brown. Remove from heat and set aside.
2. 2
   Meanwhile, in another medium pot, set over medium heat, add two tablespoons of olive oil. When warm, add the mushrooms and mix, cooking for 5 to 7 minutes, until softened and golden brown. Sprinkle with a pinch or two of salt and remove from the pot. 
3. 3
   To the same medium pot (no need to wash it out), add the butter. When the butter has melted, add the thyme leaves and flour and mix it together and cook the flour butter mixture for about 1 minute. Pour in the milk and broth in two batches, stirring continuously in between additions. Cook until it thickens up, about 2 to 3 minutes. It’ll start to bubble up and thicken. Give it a good whisk and be sure to get in those corners of the pan to get any floury bits up and mixed in. It should be thick enough to coat the back of a spoon. 
4. 4
   Turn off the heat and whisk in 1 1/2 teaspoons of kosher salt, crushed red pepper, Dijon mustard and a few rounds of pepper. Give it a taste and adjust the salt to taste. Add the grated cheeses and mix them in until they’re melted. Add the reserved caramelized onions and mushrooms. Cover the pot with a lid and set aside. 
5. 5
   Meanwhile, bring a pot of salted water to a boil. Cook the pasta according to the package’s instructions, about 9 minutes. 
6. 6
   Drain the pasta and then immediately add it to the pot with the cheesy mixture; toss until thoroughly combined. 
7. 7
   Preheat oven to 375 degrees F. 
8. 8
   In a medium saucepan, add the butter. When melted add the cubed bread and toss until combined. Add a pinch of two of salt. Pour on top of the pasta bake and transfer to the oven to bake for about 15 to 20 minutes, until the top is lightly golden brown.