Preheat oven to 375F; generously grease a 9 by 13-inch casserole dish with butter.
Cook the pasta shells to al dente according to the package directions in salted water. (I like to add a splash of olive oil to the shells as they cook to help prevent them from sticking to together since shell-shaped pasta can is pretty delicate and once it breaks is nearly impossible to stuff.)
Melt 1 tablespoon butter in a medium skillet over medium heat; once hot, add the onion, 1/4 teaspoon salt, and 1/4 teaspoon black pepper and cook until softened and just starting to brown, about 5 minutes, stirring occasionally.
Add the garlic, Italian herb seasoning, and nutmeg, and cook 1 minute more, stirring constantly. If any browned bits have formed on the bottom of the pan, add a splash of water and scrape them up with a wooden spoon. Turn off the heat and cool slightly.
Combine the onion mixture, ricotta cheese, pumpkin puree, Cheddar, Parmesan, and eggs in a large bowl.
Fill the shells with the cheesy pumpkin filling (be careful not to fill them too full or the filling will ooze out when cooking), and arrange the shells in the bottom of the prepared dish. Cut the remaining 1 tablespoon of butter into small pieces and dot the tops of the shells with it.
Cover the dish and bake until the cheese is melted, about 30 to 35 minutes. Remove the cover during the last 5 minutes so the shells can brown a little.
Serve hot, with fresh minced parsley sprinkled on top.