After preparing broccoli and cauliflower, boil or steam in pot of water 4-5 minutes, until semi-tender. Drain and place in lightly greased 9x13 inch pan.
In a large skillet, melt the butter and cook the onion and garlic until tender and fragrant, about 4-5 minutes. Whisk in the flour until thick, then gradually whisk in the milk. Keep whisking until mixture is smooth and creamy. Season with salt, pepper and fresh thyme.
Pour creamy mixture over broccoli and cauliflower, then sprinkle on the cheddar and parmesan cheese.
In a small bowl, combine bread crumbs, garlic powder, and melted butter. Spread over broccoli cauliflower casserole.
Bake at 350° 15-20 minutes until top is golden brown and broccoli and cauliflower are tender.
Notes
Use fresh broccoli and cauliflower for the best texture.Can be prepared up to 48 hours ahead of time, then baked before serving. Bake until bubbly in the center. RECIPE UPDATE (10/6/2025): The chicken broth (1 cup) has been removed from the cream sauce that is poured over the broccoli and cauliflower before baking to create a more thickened sauce in the final dish. If your sauce is too thick without the chicken broth, feel free to add a little more milk to thin it to your liking.