Rimmed baking sheet lined with parchment paper or foil sprayed with nonstick cooking spray.
Preheat oven to 425°F.
Heat butter or oil in medium saute pan over high heat. Add bananas and saute until golden. Remove the bananas to a bowl and add onion to saute pan. Saute 3 to 5 minutes until clear and soft, remove to bowl. Add beans to bowl. Using back of fork or potato masher, mash bean filling to coarse paste; season with salt and cool.
Unfold each puff pastry sheet on floured cutting board. Cut each into 9 squares.
Place 1 heaping tablespoon of cheese in the center of each square. Place one tablespoon of filling in center on top of the cheese.
Fold 1 corner of dough over filling to opposite corner, forming a triangle. Using a fork, seal the edges of the dough by pressing down around edges, taking care not to poke holes in the dough. Arrange on rimmed baking sheet and brush with egg wash.
Bake triangles until golden brown and puffed, about 20 minutes.