Preheat the oven: Preheat your oven to 375°F (190°C).
Prepare the vegetables: In a large pot, bring water to a boil and blanch the broccoli, cauliflower, and carrots for 3-4 minutes until they are tender but still firm. Drain and set aside. If using frozen veggies, just thaw them and set aside.
Cook the aromatics: In a large skillet, heat the olive oil and butter over medium heat. Add the chopped onion, bell pepper (if using), and garlic. Sauté for about 5 minutes until the vegetables are soft and fragrant.
Make the sauce: Add the flour to the skillet with the sautéed vegetables and stir to create a roux. Gradually add the milk or cream, whisking constantly, until the mixture thickens into a creamy sauce. Stir in the salt, pepper, and dried thyme.
Combine the vegetables: Add the cooked vegetables (broccoli, cauliflower, carrots, corn) to the skillet with the creamy sauce. Stir to combine.
Assemble the casserole: Transfer the vegetable mixture into a greased baking dish (about 9×9 inches). Sprinkle the shredded cheese evenly over the top.
Add the topping: In a small bowl, mix the breadcrumbs with a tablespoon of melted butter. Sprinkle this mixture over the casserole for a nice crispy topping.
Bake: Bake the casserole in the preheated oven for 20-25 minutes, or until the cheese is bubbly and the breadcrumbs are golden brown.
Serve: Allow the casserole to cool for a few minutes before serving. Garnish with fresh herbs like parsley or thyme if desired.
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