Toss the pumpkin cubes with olive oil and salt. Roast at 350 degrees for 45 minutes; set aside. Sauté the onions in butter until golden. Add the risotto and cook until lightly brown; season with salt and pepper. Add the chicken stock 1/2 cup at a time, stirring constantly until the stock is absorbed before adding more stock. The risotto should be creamy and slightly al dente. Fold in the roasted pumpkin and sage, and finish the dish with butter and Parmesan cheese.