Portion the skate by removing the flesh from the bone and cutting into four. Reserve in the fridge. Reduce the nage down by half, add a drop of cream and incorporate the cold butter. Season with salt, pepper and a little lemon juice. Fry the skate in a hot pan with a little oil. At the same time cook the spinach with the clove of garlic and the butter. Season. Place the spinach in a bowl and put the skate on top, after seasoning with salt and a little lemon juice. Keep warm. For the mussel tartare: Steam the mussels in a dash of wine in a tightly covered pan until they open. Turn out of the pan and allow to cool a little. Meanwhile blanch the onions briefly in boiling water then drain and refresh in cold water. Pat dry. Remove the mussels from their shells and place in a bowl with their finely strained juices. Add the capers, onion, gherkin, tomato and parsley to the nage. Adjust the seasoning again with salt and lemon juice, then add to the mussels with their liquid. heat through, but do not boil. Pour the mussel tartare garnish and sauce around the skate and serve.