In a large skillet, cook the ground beef on medium-high heat for 3-5 minutes, until slightly pink. Add the diced onions and salt and pepper to taste. Cook for an additional 2-3 minutes, until the onions are softened and the ground beef is cooked completely. (If you are using a ground beef that has a higher fat content, drain the fat from the skillet before adding the onions.) Remove from skillet and set aside in a warm bowl.