Step 1: Preheat the Oven

Preheat your oven to 375°F (190°C). Grease a large casserole dish with olive oil or butter to prevent sticking.

Step 2: Prepare the Vegetables

Wash and chop all the vegetables. In a large skillet, heat 1 tablespoon of olive oil over medium heat. Add the chopped onion and garlic and sauté for 2-3 minutes until softened and fragrant.

Step 3: Cook the Vegetables

Add the broccoli, cauliflower, carrots, zucchini, and red bell pepper to the skillet. Season with salt, pepper, and dried thyme. Cook for another 5-7 minutes, stirring occasionally, until the vegetables are just tender but still vibrant.

Step 4: Make the Yogurt Mustard Sauce

In a small mixing bowl, whisk together the Greek yogurt, Dijon mustard, whole grain mustard, lemon juice, olive oil, and honey or maple syrup (if using). Add salt and pepper to taste. The sauce should be smooth and creamy with a tangy flavor.

Step 5: Combine and Assemble

Transfer the cooked vegetables to the prepared casserole dish. Pour the yogurt mustard sauce over the vegetables and mix everything gently to coat evenly.

Step 6: Prepare the Topping

In a small bowl, mix the breadcrumbs, grated Parmesan cheese (if using), and olive oil or melted butter. Sprinkle this mixture evenly over the top of the casserole to create a crispy, golden crust.

Step 7: Bake the Casserole

Place the casserole in the preheated oven and bake for 20-25 minutes, or until the top is golden and crispy, and the sauce is bubbling. If you want a darker crust, you can broil the casserole for the last 2-3 minutes of baking, but be careful not to burn the topping.

Step 8: Serve

Once baked, remove the casserole from the oven and allow it to cool slightly before serving. This dish pairs wonderfully with a side of crusty bread or a light salad.