Place a rack in bottom third of oven; preheat to 400°. Toss 1 large red onion, thinly sliced, 6 garlic cloves, finely grated, one 17.5-oz. package shelf-stable potato gnocchi, ⅓ cup extra-virgin olive oil, 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, ½ tsp. crushed red pepper flakes, and ½ tsp. freshly ground black pepper on a rimmed baking sheet to combine. Create a little open space on one side. Finely grate zest of 2 lemons over gnocchi mixture; cut lemons into wedges and set aside for serving. Crumble 12 oz. spicy bulk Italian sausage in about 1" pieces onto open space on baking sheet. (It’s okay if some pieces lay on top of gnocchi and onion.)

Bake, tossing halfway through to evenly coat everything in oil again, until sausage and gnocchi are browned and crisp, 35–45 minutes.

Add 1 medium bunch broccoli rabe (about 8 oz.), cut into 2" pieces, to baking sheet and toss with tongs or 2 flat metal spatulas to coat in oil. Bake until broccoli rabe is tender, 6–10 minutes.

Remove gnocchi mixture from oven and scatter reserved lemon wedges around. Top with finely grated Parmesan, if desired.