Place pumpkin halves cut side down in a large roasting pan with the chopped parsnips nestled in the hollows where the pumpkin innards had been. Add about 1/4″ (6 mm) water to the roasting pan. Bake at 300°F (150°C) for about one hour. Remove from oven and let cool to room temperature. Scrape pulp from pumpkin rind, mash parsnip pieces, and set aside.
Heat strike water to achieve a mash rest temperature of 152°F (67°C) and add to mash tun. Add pumpkin, parsnips, and rice hulls. Stir well. Mash in grains. Rest at 152°F (67°C) for 90 minutes.
While the mash rests, lightly toast the walnuts at 325°F (163°C) for about 12 minutes. Let nuts come to room temperature and crush in a paper bag (this will help remove some of the oils). Set aside.
Lauter and sparge to collect 6.5 gal. (24.6 L) of wort. Boil 90 minutes, adding hops, walnuts, and Whirlfloc as indicated above. Chill to desired fermentation temperature, pitch yeast, and ferment until specific gravity stabilizes at or near 1.009 (2.3°P). Bottle or keg.