1. Pulse bread in food processor until coarsely ground. Heat 2 tablespoons oil in 12-inch straight-sided sauté pan over medium-high heat. Add bread crumbs and cook, stirring frequently, until beginning to brown, about 3 minutes. Stir in 1 tablespoon garlic; cook, stirring constantly, until garlic is fragrant and bread crumbs are dark golden brown, about 1 minute. Season bread crumbs with salt, transfer to small serving bowl, and set aside. Wipe out pan with paper towels.

2. Heat remaining tablespoon oil in now-empty pan over medium-high heat, add pancetta, and cook until crisp, about 8 minutes. Transfer with slotted spoon to small bowl.

3. Add onion to pan; cook until starting to brown, about 5 minutes. Add remaining tablespoon garlic and red pepper flakes; cook, stirring constantly, until garlic is fragrant, about 30 seconds.

4. Add half of greens to pan; using tongs, toss occasionally, until starting to wilt, about 2 minutes. Add remaining greens, broth, and 3/4 teaspoon salt; cover (pan will be very full); increase heat to high and bring to strong simmer. Reduce heat to medium and cook, covered, tossing occasionally, until greens are tender, about 15 minutes (mixture will be somewhat soupy). Stir in beans and pancetta.

5. Meanwhile, bring 4 quarts water to boil in Dutch oven over high heat. Add spaghetti and 1 tablespoon salt; cook until pasta is just shy of al dente. Drain pasta and return to pot. Add greens mixture to pasta, set over medium-high heat, and toss to combine. Cook until pasta absorbs most of liquid, about 2 minutes. Add Fontina; adjust seasoning with salt and pepper. Serve immediately, passing garlic bread crumbs separately.

Serves 4 to 6