1 lb. Rigatoni, Ziti, or Fusilli
8 tbsp. Extra-Virgin Olive Oil, divided
3 large Leeks, white and pale green parts only, halved lengthwise, thinly sliced into half-moons
Freshly ground Black Pepper
1 tsp. (or more) crushed Red Pepper Flakes
2 bunches Broccoli Rabe , or 3 bunches baby broccolini, trimmed, coarsely chopped
1 cup Heavy Cream
12 oz. Sharp White Cheddar, coarsely grated (about 3 cups), divided
1 cup chopped Chives, divided
1 cup coarse fresh Breadcrumbs or Panko (Japanese breadcrumbs)
Preheat oven to 425°. Bring a large pot of water to a boil, then add a generous amount of salt. Cook pasta, stirring occasionally to keep it from sticking together, until just barely al dente, about 2 minutes less than package directions. Drain pasta, reserving 1 cup pasta cooking liquid.
While you are cooking the pasta, get the broccoli rabe going. Heat 5 Tbsp. oil in a large Dutch oven or other heavy pot over medium–high. Add leeks and season with salt and black pepper. Cook, stirring occasionally, until leeks are softened (but not so much that they don’t have any texture left) and starting to brown, 8–10 minutes.
Add red pepper flakes to leeks and stir to incorporate. Add broccoli rabe by the handful, stirring to combine and allowing each addition to wilt before adding more. Season with salt and black pepper. Once all of the broccoli rabe has been added, cook, stirring occasionally, until bright green and wilted, about 2 minutes. Remove pot from heat and set aside.
Add pasta to reserved broccoli rabe mixture along with cream, three-quarters of the cheese, ½ cup chives, and reserved pasta cooking liquid; mix well. Season with salt and black pepper and add more red pepper flakes if you prefer more heat (keep in mind that the saltiness and spiciness will increase as the pasta bakes).
Transfer pasta to a 3-qt. baking dish (or, if your Dutch oven is ovenproof, just leave it in there). Toss breadcrumbs and remaining 3 Tbsp. oil in a medium bowl until coated; season with salt and black pepper. Scatter over pasta, then sprinkle evenly with remaining cheese. Bake until pasta is bubbling across the entire surface and breadcrumbs are deep golden brown, 30–35 minutes. Let cool slightly.
Scatter remaining chives over pasta just before serving.