Cook 2 bunches chopped broccoli rabe in boiling salted water until tender, 5 minutes; drain. Heat 2 tablespoons olive oil with 2 sliced garlic cloves and a pinch of red pepper flakes in a skillet over medium-high heat. Add 1/2 pound sliced shiitake mushroom caps and cook until golden, 2 minutes. Add 2 more tablespoons oil and the broccoli rabe; cook, stirring, 5 minutes. Season with salt.