Preheat oven to 400 degrees F. Butter an 8x8 casserole dish and set aside.
Bring a large pot of water to a rolling boil; add the broccoli and cauliflower and blanch for 1 minute, then drain well.
Melt butter in a deep skillet over medium heat. Saute onion 3-4 minutes, then add the garlic.
Stir in the cream cheese until melted, and then stir in the heavy cream, mustard, salt, pepper, half of the cheddar, and the mozzarella. Once the cheese is melted, stir in the broccoli & cauliflower.
Pour the contents into the prepared dish and top with the remaining cheddar. Bake uncovered 20-25 minutes, until cheese is bubbling and golden.
You can use all broccoli or all cauliflower instead of a mix.
Cream Cheese: This helps give this dish the creamy cheesy texture. You can substitute for mascarpone or even cottage cheese, but the texture and flavor of the casserole will be different.
Heavy Cream: Use whole milk instead, if needed.
Store in the refrigerator covered for up to 3 days.
To freeze, store the uncooked casserole in a freezer safe container, making sure it is wrapped well or has an airtight cover. Store for up to 3 months. Then bake the casserole at 400 degrees for 30 to 35 minutes, or until cooked through.
To reheat, cover with foil and heat in the oven at 350 degrees F. Reheat for 20 to 25 minutes or until heated through.