In a large skillet, heat the oil over medium-low heat. Add the onions and cook until lightly golden and soft, about 4-5 minutes. Stir in the garlic and cook for 30 seconds. Stir in the beef and cook until browned; season with salt and pepper. Stir in the tomato paste or marinara sauce and tomatoes and mustard. Allow to cook on medium heat for about 5 minutes.