Combine the squash, onion, water, and salt and pepper to taste in a small saucepan. Bring to a boil, reduce the heat and simmer until the squash is tender, 5-6 minutes.
Heat the olive oil and butter in a very large skillet until very hot but not smoking. Add the sage leaves a few at a time and fry until crispy, 1 minute. Use a slotted spooon to lift the sage leaves from the pan, drain them on paper towels, and season them with salt and pepper; reserve the oil-butter combination.
Cook the tortelloni according to the package instructions and drain, reserving 1/2 cup of the pasta cooking water. Add the pasta to the sage butter along with the pumpkin and celery. Toss with the cheese, adding pasta cooking water a little at a time to loosen the pasta if needed. Season with salt and pepper to taste, divide among plates and top with the crispy sage leaves and additional cheese, if desired.