Preheat oven to 350 degrees. Bring a pot of salted water to a boil and add broccoli and cauliflower. Bring to a boil and drain immediately. Set aside. Heat oil in a large, deep skillet over medium heat. Add garlic and onion and cook for 5-7 minutes, until onion is translucent. Add broccoli and cauliflower and cook for 1-2 minutes. In a large bowl, whisk coconut milk, cornstarch, Dijon mustard, paprika, and cayenne together until smooth and no lumps remain. Stir into onion and vegetable mixture, bring to a boil and remove from heat when thickened (add more coconut milk or some vegetable broth, if sauce becomes too thick for your liking). Stir in nutritional yeast and cooked macaroni and transfer to a lightly greased baking dish. Bake for 30 minutes, or until broccoli and cauliflower are tender. Serve and enjoy!