Preheat your oven to 375°F (190°C).

In a large skillet, melt about 2 tablespoons of butter over medium heat. Add the diced onion and sauté for about 5 minutes until translucent and fragrant.

Stir in minced garlic and sliced mushrooms, cooking for about 4 minutes until everything is well combined and mushrooms are tender.

Add the broccoli, cauliflower, sliced carrots, and frozen peas to the skillet. Cook for 5–7 minutes, stirring frequently until the vegetables are softened but still vibrant.

In a separate bowl, whisk together a can of cream of mushroom soup, 1/2 cup of milk, and 1/2 cup of sour cream until smooth.

Combine the vegetables with the creamy mixture, stirring until every piece is evenly coated.

Grease a 9x13 inch baking dish and pour the vegetable and sauce mixture evenly into the dish.

In a small bowl, mix bread crumbs with 1/4 cup of grated Parmesan cheese. Sprinkle this mixture over the casserole.

Bake in the preheated oven for 25–30 minutes until the topping is golden brown.

Allow the casserole to cool for a few minutes before serving.