Cook the pasta according to package instructions.
Meanwhile dry-toast the pine nuts then make the pesto: put the watercress, garlic,olive oil, lemon juice, pine nuts and half of the feta in a blender, then whiz until you get smooth paste. Taste and season with salt &pepper to your liking.
Drain the cooked pasta and put it back in the pan. Add the pesto and toss well.
Serve with crumbled feta on top and plenty of ground pepper (you can add some chilli flakes, if you like). Also, you can garnish it with some watercress and/or toasted pine nuts on top.