Pastry: In food processor, pulse together flour, baking powder, sugar and salt; pulse in butter until consistency of coarse meal. Add sour cream and egg; pulse until dough sticks together when pressed. On lightly floured surface, knead lightly, just until smooth. Wrap in plastic wrap and let rest for 30 minutes. (Make-ahead: Refrigerate up to 1 day.)

Soak mushrooms in 1 cup/250 mL warm water until soft, about 30 minutes. Reserving soaking liquid, drain and finely chop.

Meanwhile, boil potatoes until tender; drain and return to pot. Over medium heat, steam excess moisture from potatoes for 1 to 2 minutes. Remove from heat and mash with sour cream; set aside.

In small skillet, melt butter over medium-high heat; sauté onion until golden brown, 6 to 8 minutes. Add garlic; sauté until fragrant, about 30 seconds. Add mushrooms, salt and pepper. Sauté until fragrant and lightly browned, 2 to 3 minutes. Pour in mushroom soaking liquid; cook until mixture is moist but no liquid pools. Scrape into potato mixture; stir together. Stir in green onions.

Roll out pastry to form 18- x 10-inch/45 x 25 cm rectangle, with long edge facing you. Leaving 1-inch/2.5 cm border at bottom, spread mushroom filling over bottom two-thirds of dough; from bottom, roll up into cylinder, pinching together long edge and 2 ends. At 1-1/2-inch/4 cm intervals, using the side of your hand in a sawing motion, divide into 12 pieces. (This should seal the edges, but pinch together any that aren't sealed.) Fold sides of each piece up to almost meet in centre; make indentation with finger in centre of each knish to form pillow-shaped rounds.

Place knishes on parchment paper-lined baking sheet; bake in centre of 350°F/180°C oven until golden brown, 40 to 45 minutes. Let cool on pan on rack. Serve warm or at room temperature.

Makes 12 knishes.