Sauté the onion for 2 to 3 minutes until it becomes translucent. Add the chopped tomatoes, green chili peppers, and the garlic; sauté for 3 minutes. Add the ground beef and cook for 10 minutes, stirring from time to time, until all the beef has browned. Reduce the heat to low; add salt and pepper to taste. Remove from the heat and let cool completely.