Break the broccoli and cauliflower into small florets. Steam for 3 to 4 minutes until the broccoli is bright green, then remove them from the heat. Set aside.
In a large skillet, melt 3 tablespoons of butter and saute the onion and garlic until the onion starts to turn translucent, about 3-4 minutes. Sprinkle the flour over the top, whisk to combine, and cook for another 45 seconds. Whisking constantly, stir in the broth, then cook until the sauce thickens. Turn heat to low and add the cream cheese, salt, pepper, and paprika. Stir to combine until the sauce is smooth. Taste and adjust seasonings - keep warm.
In a small bowl, stir together the breadcrumbs and melted butter. Set aside.
Layer half the broccoli and cauliflower in a small casserole dish. Pour on half the cream sauce, then sprinkle on half the cheese. Sprinkle a little paprika on the cheese, then repeat with another layer of broccoli, sauce, and cheese.
Cover with breadcrumbs. Bake in a 375 degrees F oven for 25-30 minutes, until cheese is melted and bubbly and the breadcrumbs are golden brown. Serve warm. Note: This can be made in advance and refrigerated until ready to bake.