STEP - 1
Place racks in center and upper third of oven; preheat to 325°. Heat a large pot of water over high. Add several tablespoons of salt and bring to a boil (this is for your pasta).
STEP - 2
Grate 12 oz. Fontina cheese on the large holds of a box grater (you should have about 4½ cups). Smash 6 garlic cloves, peel, and coarsely chop. Pick all sage leaves from stems and set aside about 10. Finely chop remaining leaves (you should have about 1 Tbsp.). Trim tough dried ends from 1 bunch of broccoli rabe, then cut stems into 2" pieces. Leave leafy ends long.
STEP - 3
Heat a deep, large, ovenproof skillet, preferably cast iron, over medium-high. Add ¼ cup oil and swirl to coat. Remove 1 lb. sausage from casings and add to skillet. Break into small pieces with a wooden spoon. Cook, undisturbed, until sausage is browned, about 4 minutes. Stir a couple of times and continue to cook, undisturbed, until sausage is fully cooked through, about 3 minutes longer.
STEP - 4
Add garlic, chopped sage, and ½ tsp. red pepper and cook, stirring, until garlic is golden, about 2 minutes.
STEP - 5
Stir in 2 cups half-and-half and simmer until sauce is thickened slightly, about 2 minutes. Reduce heat to low.
STEP - 6
Gradually add about two-thirds of cheese, bit by bit, stirring constantly and letting cheese melt completely before adding more, until sauce is smooth and thick, about 3 minutes; season with salt and remove from heat.
STEP - 7
Meanwhile, cook 1 lb. pasta shells in boiling salted water 2 minutes shy of package instructions (8–10 minutes depending on type). During the last 2 minutes, add all of broccoli rabe to pot with pasta. Drain in a colander and shake several times to remove excess water. Return pasta and broccoli rabe to empty pasta pot.
STEP - 8
Add cheesy sausage mixture from skillet to pot with pasta. Stir until pasta and broccoli rabe are coated in sauce, then transfer everything back to skillet.
STEP - 9
Cover skillet tightly with foil and bake on center rack until pasta is tender and the sauce is bubbling, 30–40 minutes. Let rest for a few minutes while you heat broiler.
STEP - 10
Remove foil and top with remaining cheese. Toss sage leaves with remaining 1 Tbsp. oil in a small bowl and arrange over pasta. Broil until cheese is browned and bubbling in spots, about 5 minutes (depending on the strength of broiler).