Cook pierogi in boiling water according to package directions.
Melt a stick of butter in a frying pan and saute onions and garlic until tender. Season to taste with salt and pepper.
Drain pierogi and saute quickly with melted butter and onion.
I like to brown them a little more with the onions. Mrs. T's package says you can saute them in butter for 8 minutes per side instead of boiling them. Since I am not a huge fan of onions I also tried them with butter and parmesan cheese, delicious!