In a large saucepan, bring 2 quarts of salted water to a boil. Add chopped broccoli rabe and cook 3 to 5 minutes. Drain and set aside.
In a large skillet heat butter and 1 tablespoon of olive oil over medium high heat. Add onion and saute until translucent, about 6 minutes. Add remaining oil, garlic, red pepper flakes, and mushrooms. Saute until golden brown, 2 to 4 minutes. Add drained broccoli rabe, cover and cook for 4 to 5 minutes or until tender, stirring occasionally. Makes 4 servings.