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 and chicken broth. 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.
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.