Melt olive oil and butter in a nonstick pan.
Saute onions and chard stems for several minutes.
Add ham and cook for an additional 2 minutes.
Add chard leaves and cook until wilted.
Remove from heat while prepping eggs and cheese.
Whisk eggs, then whisk in milk, salt and pepper.
Return pan to heat and add egg mixture.
Sprinkle the top with cheese.
Cook until set (about 4 minutes).
Then put pan under broiler for several minutes until top is browned.
Depending on the pan you use and size of your broiler, you may need to wrap handle in foil. Be very careful to use a hot mitt when removing pan from oven and when removing frittata from pan.
Let frittata sit for 1-2 minutes in pan, then run spatula around under the frittata and slide onto a plate.
Cut and serve.