In a large skillet, heat the olive oil over medium heat. Add the chopped onion and garlic, sautéing for 3-4 minutes until soft. Add the ground meat and cook until browned, about 6-8 minutes. Drain any excess fat. Stir in the cooked rice, cumin, chili powder, salt, and pepper. If using, add the diced tomatoes and cook for 2-3 minutes until everything is combined. Remove from heat and set aside.