1. Preheat Oven: Preheat your oven to 375°F (190°C).
2. Prepare the Vegetables:
   • In a large pot of boiling salted water, blanch the cauliflower, broccoli, and carrots for 3-4 minutes until slightly tender. Drain and set aside.
3. Sauté Aromatics:
   • In a large skillet, heat olive oil over medium heat. Add the finely chopped onion and sauté until softened, about 3 minutes. Add the minced garlic and sauté for an additional 1-2 minutes until fragrant.
4. Make the Creamy Sauce:
   • Reduce the heat to low and add the butter to the skillet. Once melted, stir in the sour cream, heavy cream, Dijon mustard, dried thyme, and dried oregano. Season with salt and pepper to taste. Cook, stirring constantly, until the sauce is well combined and heated through, about 2-3 minutes.
5. Combine Vegetables and Sauce:
   • In a large mixing bowl, combine the blanched vegetables, diced bell peppers, zucchini, frozen peas, and the creamy sauce. Toss gently to coat all the vegetables evenly.
6. Assemble the Casserole:
   • Transfer the vegetable mixture to a large baking dish. Sprinkle the shredded cheddar cheese and grated Parmesan cheese evenly over the top. If using, sprinkle breadcrumbs over the cheese layer for extra crunch.
7. Bake:
   • Bake in the preheated oven for 25-30 minutes, or until the vegetables are tender and the top is golden and bubbly.
8. Serve:
   • Let the casserole cool slightly before serving. Enjoy this cheesy vegetable delight as a side dish or a satisfying vegetarian main course.

Enjoy the combination of rich flavors and creamy textures in every bite!