Prep for baking. Preheat oven to 400˚F. Drizzle a little olive oil on the bottom of a square baking dish; spread it around and set aside.

Steam the vegetables. In a large skillet, bring 0.5 inch of salted water to a boil. Add the broccoli and cauliflower; cover and cook over high heat until crisp-tender, about 4 minutes. Drain the vegetables in a colander and set aside.

Sauté. Wipe out the skillet, add 2 tablespoons olive oil, and heat over medium-high heat. Add onions and cook for 2 to 3 minutes or until softened.

Add the garlic and veggies. Stir in the minced garlic and continue to stir and cook for 1 minute. Add broccoli and cauliflower to the skillet. Season with salt and pepper and continue to cook for about 2 minutes, or until lightly golden. Remove from heat and transfer the vegetables to the baking dish; set aside.

Prepare the cheese sauce. Wipe the skillet and add 1 tablespoon olive oil, 0.75 cup shredded cheddar cheese, cornstarch, evaporated milk, mustard, salt, pepper, and paprika.

Cook over low heat. Stir constantly with a whisk, until melted and thickened, about 2 minutes.

Combine. Pour the cheese sauce over the broccoli and cauliflower; stir to combine. Sprinkle the remaining 0.25 cup cheddar cheese and 0.25 cup mozzarella cheese over the top.

Bake. Bake for 20 minutes or until lightly browned and bubbly. Remove from oven and let stand 5 to 8 minutes, then serve.