sauerkraut - mushrooms pierogi

Filling for 2 sets of dough:
1 medium cut up onion fried in butter until it becomes translucent.
Add Polish dried mushrooms (first soak them for a few hours, then get as much of the water out as possible, next finely chopped them - I save this water, which I drain through the coffee filter, so I don't get any sand from my mushrooms)
[You can use canned mushrooms - finely chopped. Or you can use "Bulion grzybowy" which will give you the taste of the Polish dried mushrooms]
Add:
three cans of 14.4oz. sauerkraut (First: drain and wash the sauerkraut to reduce the sharp acid flavor - don't over-wash)
Fry all this on butter for about 5 min , then simmer (about 30 min-1 hr) adding saved water if you are using Polish mushrooms. Finally, add salt and pepper to taste.

Dough:
1 egg
1/2 cup warm milk
2 cups flour
1 tsp. salt
2 Tbs melted butter

Combine, knead & rest dough for 30 min, knead again, roll out thin 1/8", cut 3 3/4" rounds (I use my Bread & Dough Maker machine -I set to DOUGH & take out my dough after 30 min. Then we [H is helping here] use pasta machine with setting #3 - I get about 35-40 pierogi).

Add 1 tsp. filling, fold and pinch sealing. Boil pierogi for 5 min, take them out & drain.

I cover pierogi with melted butter (we like when butter is a little brown).
If we are making them in advance, then I put pierogi in a "boilable cooking bags" - this way later whenever I need them I just put the bag in a boiling water, or microwave the bag with my pierogi.

*** makes about 24 pierogi
***using pasta machine with setting #3 - I get about 35-40 pierogi