Soak the dried mushrooms in a bowl with a bit of cool water until soft. Peel the onion, rinse it and chop it finely. Rinse the sardine removing all its salt, dry it and bone it.Drain the mushrooms, squeeze and chop them coarsely. In a saucepan with some oil fry the onion until soft but not golden, mix the sardine, the finely chopped kernels chopped finely and the mushrooms.Brown them briefly, add the tomato and a glass of water; season with salt and pepper and simmer for about 10 minutes. In the meantime bring to the boil plenty of salted water toss the pasta and cook for 3 minutes.Drain it and add it in the saucepan with the mushroom and tomato mixture simmering for another 7-8 minutes, stirring from time to time with a wooden spoon. Pour the tomato pastina in the dishes and serve warm.