Thaw pastry sheets according to package directions. Saute onions in oil in a large skillet until lightly browned. Add mushrooms & cook until mushrooms are lightly browned.
Set aside. Place egg substitute in a bowl & toss the kasha in it. Place tossed kasha in a large skillet with a tightly fitting cover. Over high heat, flatten, stir & chop the kasha with a fork until the grains separate. Remove from heat.
Bring broth to a boil. Slowly pour broth over the kasha, cover skillet, & cook over low heat until the liquid is all absorbed, about 10 minutes. Remove from heat.
Stir in onion-mushroom mixture, salt & pepper. Cool to room temperature. Preheat oven to 350F & oil a cookie sheet.
On a lightly floured board, roll one pastry sheet into a 11" X 14" rectangle. Cut into 3" circles & place 2 ts filling in the centre of each circle. Pull edges up around the filling, completely enclosing it. Pinch dough together to form a tight package. Turn packages over & place seam side down on a cookie sheet.
Beat remaining egg substitute with water to make an "egg wash". Brush each knish with this wash & sprinkle with poppy seeds. Repeat with remaining sheets of dough.
Bake until golden brown, about 30 minutes. Serve warm or at room temperature.