I boil my perogies and when I take them from the water with slotted ladle, draining as well as possible, I transfer them into a large bowl which has melted butter in it.I then toss them gently to coat and place them onto cookie sheets which have been greased (shortening) to prevent the perogies from sticking. I then freeze them and put them in zip-lock bags.
When I want to use them, I take out as many as I need, drop them in simmering water, and then transfer into a serving dish with butter- fried onions, stirring carefully, or I just use melted butter to prevent sticking and serve the butter-fried onions on the side.