Heat oven to 425°F. Melt butter in 11x7-inch baking pan in oven; sprinkle evenly with sugar. Top with pecans.
Cut bananas diagonally into slices. Place bananas over sugar, overlapping if necessary. Drizzle with lemon juice.
Unfold thawed pastry; stretch or roll slightly to fit pan. Place pastry over bananas. Tuck edges of pastry in around edges of filling.
Bake for 30 to 35 minutes or until pastry is golden brown and sauce is bubbly.
Loosen edges of pan with spatula or knife. Carefully invert onto serving platter. Sprinkle with powdered sugar. Serve warm.
Make sure thawed puff pastry does not become warm or it may not rise properly. When rolling, work from center of dough toward edges taking care to keep consistent thickness.
If using a glass baking dish, reduce oven temperature to 400°F. Bake as directed above.