Fill a large pot with salted water and bring to a boil. Cook tortellini according to package instructions. Drain tortellini and set aside, but reserve 1 cup of pasta water.
Using a large deep skillet, add butter and place over medium heat. Add the onions and sauté, stirring occasionally, for 2-3 minutes.
Add garlic and continue sautéing for 1-2 more minutes.
Add pumpkin, sage and nutmeg; stir and continue sautéing for 1-2 more minutes.
Add half-and-half; stir until well combined. Once mixture begins to simmer, reduce heat to medium-low. Continue cooking for 5-6 minutes, stirring occasionally.
Add Parmesan cheese and pepper; stir until well combined. Continue cooking for 1-2 more minutes.
Add cooked tortellini and toss until well coated. (If necessary, add a bit of the reserved pasta water to loosen sauce.)
Divide onto plates and garnish with additional grated Parmesan and chopped fresh parsley.