Melt 1 tablespoon of butter in a very large skillet with a tight-fitting lid over medium-high heat. Add the sausage and cook, breaking apart the meat, until browned and cooked through, about 5 minutes. Transfer the sausage to a plate.

Reduce the skillet heat to medium. Add the remaining 4 tablespoons of butter. Once melted, add the squash and onion. Cook, stirring occasionally, until the squash is tender and golden, about 8 to 10 minutes. Add the garlic, salt, pepper, rubbed sage (if using fresh sage, add it at the end), and nutmeg. Continue cooking until the garlic is softened and fragrant, about 1 additional minute.

Add 1 1/2 cups of chicken broth and gnocchi to the skillet. Increase the heat to medium-high. Stir to coat the gnocchi, then cover the pan. Cook, covered, until the gnocchi is just tender, about 5 minutes. Uncover and stir in the sausage. Stir in the spinach a few handfuls at a time, allowing it to wilt. If a looser sauce is desired, add more chicken broth. Sprinkle with Parmesan and season to taste with additional salt, pepper, and red pepper flakes. Enjoy hot.