Step 1

Preheat oven to 425 degrees. Set a medium pot filled with salted water over high heat and bring to a boil. Add potatoes and boil until just tender, about 10 minutes. Strain and toss dry.

Step 2

Meanwhile, in a medium pan over medium heat sauté onions with 3 tablespoons melted butter until translucent, about 5 minutes. Add sausage and cook until flavors meld, about 2 minutes.

Step 3

Stir potatoes into onion-meat mixture along with 2 tablespoons melted butter. Season with salt and pepper.

Step 4

Brush an 8-inch round pan with butter and lay in three sheets phyllo, brushing each sheet with butter before adding another. Add potato-meat filling and spread out evenly. Top with 5 additional sheets phyllo, brushing each with butter. (Reserve remaining phyllo for another use.) Trim excess dough, leaving at least 1-inch overhang. Fold edges of phyllo over, rolling toward center of pie to seal. Brush top with more melted butter.

Step 5

Bake pie on center rack until golden, about 20 minutes. Meanwhile, whisk water into yogurt and season with salt.

Step 6

To serve, cut pie into wedges and garnish with feta and parsley, if using. Serve with thinned yogurt.