Heat the oil in a skillet over medium-high heat. Add the onions and sauté for about two minutes, or until they've softened and are aromatic.
Add the garlic and sausage to the skillet and cook until the sausage is mostly browned, about 5 minutes, breaking it down into crumbles with a spatula as you go.
Once the sausage is mostly cooked (you can still have a few pink bits here and there), stir in the broth, and add the tortellini.
With a spatula, nestle the tortellini down into the crumbled sausage and broth. Bring the broth to a gentle simmer, then lower the heat and cover the skillet. (Covering the skillet allows the tortellini to steam, helping them get nice and tender.)
Let the tortellini cook for as long as suggested on the packaging. Here, it was 4 minutes.
When the cooking time is up, remove the lid from the skillet and stir in the spinach, cheese, and lemon zest. Let everything cook for an extra minute or two, allowing the spinach to wilt.
Remove the skillet from the heat and add the fresh lemon juice, some cracked black pepper, and some extra cheese if you'd like. Serve immediately.