Bring a large pot of salted water to a boil. Add the gnocchi and cook until the pasta floats to the surface, 4 to 5 minutes, then drain.
In a large skillet, melt the butter over medium heat. Add the onion and sauté until translucent, 4 to 5 minutes. Add the garlic and sauté until fragrant, 1 minute more.
Add the sausage and cook until it begins to brown, 3 to 4 minutes.
Add the broccoli rabe and chicken broth, and bring to a simmer. Continue to cook, tossing frequently, until the broccoli rabe is wilted and tender, about 5 minutes.
Season with salt, pepper, and red pepper flakes (if desired).
Stir in the gnocchi, Parmesan, and parsley, and toss well until combined. Serve immediately.
