Make filling. Cook spinach in a large pan with a knob of butter for a few minutes and drain well. Cool, wring out excess water with your hands. Chop and mix with the ricotta cheese, yolks and half the parmesan. Season with nutmeg, salt and pepper. Roll out pasta onto a lightly floured surface. Make two sheets about 16 by 10 inches. Place small spoons of filling on pasta in rows, leaving space around each mound. Brush pasta with lightly beaten egg whites then position second sheet of pasta on top. Press gently to exclude air, then firmly to seal. Use a pastry wheel to cut into ravioli parcels. Carefully slide ravioli into a pan of boiling water and cook for 3-4 minutes, hoping they don't fall apart like mine do occasionally . Serve drizzled with melted butter, more nutmeg and parmesan.