In a large skillet, heat the oil over medium-low heat. Add the onions and cook until soft, about 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, then add the tomatoes and mustard. Let the mixture bubble gently until it is slightly thickened, about 2 minutes
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