1 pre-made roll polenta, sliced into 1/2-inch rounds
1 tablespoon olive oil
4 tablespoons butter, divided
3 cups mushrooms, sliced
3 cloves garlic, minced
2 tablespoons fresh thyme leaves
3 tablespoons all-purpose flour
2 cups milk
1 cup mozzarella cheese, shredded
1 cup Parmesan cheese, divided
1 teaspoon salt, divided
1/2 teaspoon ground black pepper, divided
Preheat oven to 350 F and grease a 9-by-13 casserole dish.
Place one layer of polenta rounds into the bottom of the prepared casserole dish.
Heat olive oil and 1 tablespoon of butter in a large frying pan over medium heat until melted.
Add mushrooms to the butter and oil, tossing to coat in fat. Cook for 5 to 7 minutes, until the mushrooms have lost their moisture and have begun to brown.
Add garlic and thyme leaves to mushrooms and cook for another 30 seconds.
Season mushrooms with 1/2 teaspoon of salt and 1/4 teaspoon ground black pepper.
While the mushrooms are cooking, prepare the béchamel sauce. Start by melting the remaining 3 tablespoons of butter in a medium-sized saucepan over medium heat.
Add the flour and whisk to incorporate. Allow flour to cook out for 2 minutes.
Slowly whisk in the milk, whisking out lumps as you do.
Season with remaining 1/2 teaspoon salt and 1/4 teaspoon pepper and lower the heat and allow the sauce to simmer for about 10 minutes, until slightly thickened.
Take sauce off heat and slowly add mozzarella cheese and 1/2 cup of Parmesan cheese, stirring as you do.
Spread mushrooms evenly over polenta rounds.
Pour cheese sauce over mushrooms.
Top mushrooms and cheese sauce with more polenta rounds, one round going on top of each polenta and mushroom stack.
Place in the preheated oven and bake for 45 minutes.
Remove casserole from oven and sprinkle with remaining Parmesan cheese.
Place back into the oven and turn broiler on. Broil for 2 to 3 minutes, just until Parmesan has started to brown.