Preheat oven to 400 degrees.
Heat oil in a large heavy skillet over moderately high heat until hot but not smoking and cook broccoli rabe, turning it with tongs, until wilted. Add remaining 1/4 cup water and pepper flakes and simmer, covered partially, until broccoli rabe is crisp-tender and almost all liquid is evaporated, about 2 minutes.
Spread pitas with hummus and top with broccoli rabe and Parmesan. Arrange pita pizzas on a large baking sheet and bake in middle of oven 10 minutes, or until edges are golden.
Recipe adapted from Gourmet Magazine