Cook pasta to al dente. Drain, reserving about ½ cup pasta cooking water
In a medium-sized skillet, sauté garlic and onion in olive oil for 3-5 minutes. Add half the sage and heat for another minute. Reduce heat to low and add milk and pumpkin. Stir gently to combine ingredients. Allow mixture to simmer until flavors are well combined, about 8-10 minutes. Add salt and pepper, stir to combine, and remove from heat.
Add cooked pasta and reserved pasta water. Add pine nuts (if using) and top with the remaining sage and the grated Parmesan (if desired).