Cut the beef sausage into rounds about 1/2 inch thick so the pieces cook and brown evenly.
Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add the sausage in a single layer and cook for 4–5 minutes, turning until lightly browned. Transfer it to a plate.
Reduce the heat to medium. Add the diced onion to the same skillet and cook for 3–4 minutes, stirring occasionally, until soft and translucent.
Add the minced garlic and cook for 30–45 seconds, stirring frequently, until fragrant. Do not let it turn dark brown.
Pour in 1 cup beef broth and 1 cup heavy cream. Scrape the browned bits from the bottom of the skillet and bring the mixture to a gentle simmer for about 2 minutes.
Stir 1 pound potato gnocchi into the sauce and make sure the pieces are separated and coated.
Cook at a gentle simmer for 5–7 minutes, stirring every minute or so. Return the browned sausage during the final 2 minutes. If the sauce becomes too thick before the gnocchi is tender, add 2–3 tablespoons beef broth.
Reduce the heat to low. Gradually stir in 1/2 cup finely grated Parmesan and cook for 1–2 minutes, until melted and creamy. Season with 1/4 teaspoon black pepper and optional 1/4 teaspoon red pepper flakes.
Add 2 cups baby spinach and gently stir for 1–2 minutes, until wilted.
Remove the skillet from the heat and let the gnocchi rest for 2–3 minutes. If the sauce thickens too much, stir in 1–2 tablespoons warm broth or cream. Serve warm with extra Parmesan or parsley if desired.
