Meanwhile, in a large, deep skillet, heat the remaining 3 tablespoons of olive oil until shimmering. Add the pancetta and cook over moderately high heat until golden, 4 to 5 minutes. Add the onion, garlic and crushed red pepper and cook until the onion softens. Add the broccoli rabe and the remaining 1/4 cup of water, season with salt and pepper and toss gently. Cover and cook until softened, 5 minutes. Uncover and cook, stirring occasionally, until the water has evaporated and the broccoli rabe is tender, 2 minutes longer.