For the sauce, melt a tablespoon of butter, add 1 tablespoon of flour and brown briefly. Deglaze the whole thing with 1 cup of cream and 1/2 cup of water (if necessary). Add some tomato paste to the sauce and stir everything well.
Clean the mushrooms and cut into slices. Add the mushrooms, some spinach and the shrimps to the sauce and simmer for about 2 - 3 minutes.
In the meantime, cook the tagliatelle in plenty of salted water for about 8 minutes and then drain well. Mix the hot sauce with the hot noodles. Season with onion salt and coarse pepper.