Destalk the swiss chard, wash and chop the leaves. Save the large white stalks for another use. Place swiss chard leaves in a pot and add a pinch of salt. Cook over medium heat until leaves are tender, about 10 minutes. Drain any liquid from the chard.
While swiss chard is cooking, cook onion and olive oil in a skillet until translucent. Add cooked chard to skillet and cook 2-3 minutes or until liquid evaporates.
Combine swiss chard mixture, ricotta, egg yolk, parmesan and nutmeg in a bowl.
Place rolled out pasta dough onto counter. Fold and cut in half on short side, so you have two equally sized pieces of dough. Brush with water and place a teaspoonful of swiss chard mixture onto pasta dough two across, leaving two inches of space between the mounds. Place remaining half of dough on top of it and beginning at center, squeeze out any air, as you seal the mounds into ravioli shapes. Cut with zigzag cutter into squares. Flour each ravioli and place on parchment or wax paper, so they won't stick together. Raviolis can be frozen for later use if you wish.
When sauce is ready, add raviolis to boiling water. Cook only a minute until they float to the top. Lift out with skimmer and drain.