Adjust oven rack to lower-middle position, and place a rimmed baking sheet on oven rack. Preheat oven to 425°F (220℃).
In a large bowl, toss together gnocchi, onion, 2 tablespoons oil, 1 teaspoon salt, red pepper, fennel seeds, and black pepper until gnocchi is evenly coated.
Remove baking sheet from oven, and spread gnocchi mixture in an even layer on preheated baking sheet. Place sausage pieces directly on baking sheet between gnocchi.
Bake until sausage is cooked through and both sausage and gnocchi are browned and crisp, about 30 minutes, stirring halfway through baking time.
In same large bowl, add broccoli rabe, cherry tomatoes, garlic, lemon zest, remaining ½ teaspoon salt, and remaining 2 tablespoons oil; massage oil into broccoli rabe until evenly coated.
Spread broccoli rabe mixture over gnocchi mixture. Bake until broccoli rabe and tomatoes are tender, 6 to 10 minutes.
Remove baking sheet from oven, and stir in lemon juice and Parmesan. Top with additional Parmesan, and serve immediately.
