Warm the skillet: Heat a large, deep skillet over medium heat.
Add a drizzle of olive oil and the butter. When the butter foams, add the onion with a pinch of salt. Cook 3–4 minutes until softened.
Add garlic and sun-dried tomatoes: Stir in the garlic, sun-dried tomatoes, Italian seasoning, and a pinch of red pepper flakes if using.
Cook 30–60 seconds until fragrant. Don’t let the garlic brown.
Deglaze: Pour in the broth and stir, scraping up any browned bits. Bring to a gentle simmer.
Add cream and tortellini: Stir in the cream, then add the tortellini.
Bring back to a simmer. Reduce heat to medium-low and cook 4–6 minutes, stirring gently, until the tortellini are tender and floating.
Fold in protein (optional): If using cooked chicken or sausage, stir it in now and warm through for 1–2 minutes.
Finish the sauce: Sprinkle in the Parmesan a handful at a time, stirring until melted and smooth. If the sauce feels too thick, add a splash of broth.
If it’s thin, simmer another minute.
Add greens: Fold in the spinach and cook until just wilted, about 1 minute. Taste and season with salt and pepper. Add a squeeze of lemon juice or a little zest if you want a brighter finish.
Serve: Top with extra Parmesan and chopped basil or parsley.
Serve hot straight from the skillet.