Mix the butter with the ingredients. Bring 1dl of water to a boil in a spacious pan. Put the mussels in the cooking water and put the cover on the pan. Cook them briefly till they open. Then pour them in a dish so that they can cool down quickly. Remove the empty shell of each mussel and cover the mussels with a small spoon of the tomato butter. Heat them on the plate till the butter has melted.