Cook the brown rice according to package instructions. Set aside.
In a large skillet over medium heat, brown the beef until cooked through. Drain and set aside in a glass dish.
Return the skillet to medium heat and add the butter. Add the yellow onion, bell pepper and garlic and cook until the onion is translucent.
Add the tomatoes, green chilies, salt and pepper. Stir the beef back in.
Fold in the cooked rice, cheddar and Monterrey jack cheese.
Stir until the cheese is melted, then serve with avocado slices, cilantro and green onions.