In a food processor, combine flour and salt, and blend. Add water, eggs and butter. Pulse until dough forms into a ball. Add one to two tablespoons of water if mixture is lumpy. Rest for 20 minutes.

Divide dough into four parts. On a floured surface, roll one portion of dough to 1/8-in. thickness, and place filling in each pocket, folding over the top and pressing the edges to seal filling in. Repeat with remaining dough.

Place potatoes in a large saucepan and cover with water. Bring to a boil over high heat. Reduce heat; cover and simmer until tender, 10-15 minutes.

In a large skillet over medium-high heat, saute onions in butter until tender.

Drain potatoes. Over very low heat, stir potatoes for 1-2 minutes or until steam has evaporated. Stir in cream cheese, salt, pepper and onion mixture; set aside.

Bring a skillet of water (cast iron preferred) to a boil over high heat; add pierogies.