In a saucepan, cook the onion in the oil over moderate heat, stirring, until just softened, about 5 minutes. Stir in the tomato paste and cook for 1 minute. Add the potatoes, bouillon, crushed pepper and 5 cups of water. Cover and bring to a boil. Stir in the pasta and cook until al dente, about 10 minutes. Stir in the tomato and herbs and cook for 1 minute longer. Season with salt and serve with mascarpone.