Trim the stems of the watercress and herbs then put the leaves into a small blender with the garlic, capers and anchovies and whiz until finely chopped. (You could do this by hand, but you would need a little patience.) Add the butter, lemon juice and pepper and blend until evenly mixed. Scoop out and place in a serving bowl.
Heat a large, lightly oiled roasting tray in the oven for 5 minutes.
Pat the skate wings dry with kitchen paper to remove excess moisture, then lay them on the heated roasting tray. Drizzle with a little olive oil and