In a saucepan, over high heat, heat the oil. Brown the beef cubes on all sides. Add the onion and garlic. Continue cooking for 1 minute, stirring constantly. Add the tomato paste and broth. Season with salt and pepper. Bring to a boil. Over low heat, simmer for 20 minutes. Meanwhile, cut the bell peppers in half, seed and slice into strips. In the saucepan, add the vegetables. Continue cooking for 5 minutes. Add the cornstarch diluted in water, stir to combine. Continue cooking for 3 minutes, stirring occasionally. Serve.