Creating the perfect Baked Cheesy Mushrooms Casserole is straightforward. Follow these simple steps:

1. Preheat Oven: Preheat your oven to 350°F (175°C).
2. Sauté Vegetables: In a large skillet, heat olive oil over medium heat. Add chopped onions and minced garlic, sautéing until translucent.
3. Cook Mushrooms: Add sliced mushrooms to the skillet. Cook until they release their moisture and become tender, about 5-7 minutes.
4. Mix Ingredients: In a large bowl, combine the sautéed mushrooms, sour cream, thyme, salt, and pepper. Stir until well mixed.
5. Add Cheese: Fold in 1 1/2 cups of shredded cheese into the mushroom mixture, reserving the remaining cheese for topping.
6. Prepare Casserole Dish: Grease a baking dish (approximately 9×13 inches) and pour the mushroom mixture into it, spreading it evenly.
7. Top with Breadcrumbs: Sprinkle breadcrumbs evenly over the top of the casserole.
8. Add Remaining Cheese: Top the casserole with the reserved 1/2 cup of shredded cheese and grated Parmesan cheese.
9. Bake: Place the casserole in the preheated oven and bake for 30-35 minutes or until the top is golden and bubbly.
10. Garnish: Remove from the oven and let it cool slightly. Garnish with fresh parsley before serving.

Following these steps will ensure that your Baked Cheesy Mushrooms Casserole turns out perfectly every time.