For the anchovy butter, cut the anchovies into large pieces and mash them with a fork. Mix the softened butter with lemon juice and stir in the anchovies.
For the tomato sauce, cut the onion into small cubes and sweat in olive oil until translucent. Add the coarsely chopped tomatoes with their juice and allow to reduce a little. Season to taste with salt and pepper. Do not use herbs, they would mask the slightly nutty taste.
Meanwhile, cook the spaghetti in salted water until al dente, strain and return to the saucepan, mix with the anchovy butter. Pour immediately onto 2 plates, add the sauce and sprinkle with parmesan.