Pierogi with Butternut Squash and Sage

A fun main course for autumn or winter. The pierogi dough is extremely easy to work with. Ingredients listed below:
The butternut squash filling:
A 3-lb (or slightly smaller) butternut squash
5 large or 10 small sage leaves
Salt and pepper to taste
For the pierogi dough:
2 cups all-purpose flour
3/4 cup sour cream
1 large egg
1/4 cup olive oil
1/2 teaspoon salt