Unroll phyllo dough sheets one sheet at a time on flat surface. Brush with unsalted butter. Cut into strips about 5 inches wide. Place 1 tablespoon of the onion, spinach and feta mixture at one end of each dough strip. Fold the end of the strip over the filling so that it forms a triangle. Continue folding the strip in triangles until a small, triangular stuffed pastry remains. Brush with unsalted butter. Repeat with process until all of the phyllo dough is used.