In a large skillet, melt butter or heat olive oil. Sauté onions and garlic, stirring occasionally, until onions are wilted and just browning on edges.
In a large bowl, fork beat eggs. Stir in milk. Add dry mustard, nutmeg, salt. Mix the cheeses together but save 1/2 cup to sprinkle on top of the casserole.
Add cheese, onions, rice, and beans. Mix together.
Grease a 9x9 casserole pan with butter or olive oil. Turn the rice, cheese, and bean mixture into the pan. Sprinkle 1/2 cup of cheese on top.
Bake at 375F for 40 minutes. Let it cool for about 10 minutes before serving.
Note: You can substitute red kidney beans for black beans and feel free to use whatever cheese you have on hand. Colby, Monterey Jack, Fontina, Mild Cheddar, Pepper Jack are all fine.