Preheat oven to 350° F. Bring water to a boil. Add pasta and undercook by 2 minutes.
In a separate covered pot, steam broccoli and cauliflower florets for 5 minutes or until tender. Remove and set aside. Discard liquid from pot.
Finely chop onion and garlic cloves. Using the same pot, over medium heat, melt 3 tablespoons butter then add onion and garlic. Sauté for 3-4 minutes, until tender. Add flour and whisk for one minute. Whisk in rice drink and season with pepper and nutmeg. Stir until thickened, then stir in the spicy mustard.
Mix all three cheeses together. Add 2/3 of the combined cheeses to the sauce and stir until cheese is melted.
Drain pasta. Add cauliflower and broccoli then stir in the cheese sauce.
Spray a 9” x 12” baking pan with nonstick cooking spray and pour in the pasta mixture. Top with the remaining combined cheeses.
Place baking pan in the oven on top of a baking sheet to catch bubble over. After 30 minutes of baking, increase heat to 400° F. Sauté remaining 1 tablespoon butter with 1 cup panko bread crumbs and add to top of casserole. Cook additional 8 to 10 minutes until bread crumbs are browned.