Heat 2 tablespoons oil in heavy large skillet over medium-high heat. Add garlic and cook until golden brown, stirring frequently, about 3 minutes.
Discard garlic. Add onion, pancetta and fennel seeds to skillet; sauté until onion is tender and pancetta begins to brown, about 8 minutes. Add crushed red pepper (if using), then broccoli stems and cook 4 minutes to soften slightly, stirring occasionally. Stir in broccoli tops, sprinkle with salt, and add 1 cup water. Cover and cook until stems and tops are tender, about 5 minutes. Season to taste with salt and pepper.
Meanwhile, cook pasta in large pot of boiling salted water until just tender but still firm to bite, stirring occasionally. Drain pasta, reserving 1 cup cooking liquid.
Add pasta to skillet with broccoli and stir over low heat to combine, adding reserved cooking liquid by tablespoonfuls to moisten if necessary. Stir in remaining 2 tablespoons oil and 1 cup cheese. Season to taste wtih salt and a generous amount of pepper.
Transfer to large shallow bowl. Sprinkle with pine nuts and serve, passing additional grated cheese separately.