Preheat the oven to 350 degrees
Cut the chard leaves by running a knife down the center along each side of the stem. Put the leaves in one pile and the stems in another. Chop all the stems into 1/2 inch slices, keep separate. Roughly chop the leaves, keep them separate.
Dice the onion.
In a 10-inch oven-safe skillet, warm the olive oil on medium heat, then add the chopped stems and onion. Cook until the onions and stems are soft (about 5 minutes).
Meanwhile, crack the eggs into a medium sized bowl and beat them with a fork. Stir in the cheese.
To the skillet, add the chopped chard leaves. Cook until they are wilted, about 2 minutes.
Add the egg/cheese mixture to the pan. Move around the ingredients so that everything is evenly distributed.
Move the pan to the oven. Cook the frittata for 15 minutes. When the frittata is cooked through, remove it from the oven and allow it to cool for a few minutes before serving.