Hit SAUTE on the Instant Pot, and when it reads Hot, add 1 tablespoon butter, chopped onions and kielbasa
Cook until onions are slightly soft, then hit CANCEL
Add frozen pierogi over kielbasa, then pour in water, and add remaining butter on top
Place lid on pressure cooker, make sure vent is set to Sealing position, and hit PRESSURE COOK for 1 minute
Pot will take a few minutes to reach pressure, then cook for 1 minute. Do a quick release, and when pin drops, remove lid, and hit CANCEL
Pour in the frozen peas and heavy cream, then hit SAUTE and add cheese, 1 cup at a time, stirring VERY gently
When liquid starts bubbling, hit CANCEL to turn pot off, and serve. Optional -- To help cool down, using hot mitts, carefully lift the inner pot out and pour contents into lightly greased casserole dish.
Liquid will thicken as casserole cools. To make a baked casserole, layer more cheddar cheese with the pierogi and kielbasa, top with additional cheese, and broil in oven until top is crispy