Preheat oven to 350˚F and grease 6 cups of the muffin tin with butter. Skip this step if you are baking the frittata directly in muffin paper cups.
Put remaining butter in skillet and melt with medium heat. When butter is melted, add onions and garlic to skillet. Cook until onions are translucent or no longer crunchy, stirring occasionally. If butter is browning too fast, lower heat.
Add chopped chard to skillet. Cook for a few minutes until it is wilted. Remove from heat and set aside.
Crack 5 eggs into a bowl and whisk lightly. Add a tablespoon or two of water if you want to make the eggs extra fluffy.
Add cheese and chard to eggs and mix so that ingredients are evenly distributed.
Pour egg mixture into muffin tin so that each cup is full. Put in preheated oven for 15 minutes or until top of frittata is just cooked.
Remove from oven and carefully pry them out of muffin tin to cool. These will fluff up and shrink down again once they out of the oven.