Preheat your oven to 350°F (175°C). Gather all your ingredients and tools.

In a large pot, bring salted water to a rolling boil. Cook broccoli and cauliflower florets for 5 minutes until tender-crisp.

Quickly drain the vegetables and plunge them into ice water to stop further cooking.

In a mixing bowl, combine sour cream, mayonnaise, Dijon mustard, garlic powder, and onion powder. Stir in half of the cheddar cheese until smooth.

Gently fold the drained broccoli and cauliflower into the creamy mixture until coated.

Transfer the vegetable mixture into a greased 9x13-inch casserole dish, spreading it evenly.

In a separate bowl, mix breadcrumbs, melted butter, remaining cheddar, and Parmesan cheese until moist and crumbly.

Evenly sprinkle the breadcrumb mixture over the casserole and bake for 25–30 minutes until golden and bubbly.

Let the casserole rest for a few minutes before serving.