Halve the squash, scoop out the seeds, and place it cut side down on an oiled sheet pan.
Roast at 400°F until tender when pierced with a paring knife.
With a metal spoon, scrape out the cooked flesh and puree in a blender or food processor or mash with a potato masher until smooth.
Preheat oven to 350°F. Oil a loaf pan. In a large bowl, stir together flours, baking soda, salt, cinnamon, allspice and nutmeg.
In a medium bowl, whisk the squash puree, brown sugar, eggs, yogurt and oil until smooth. Pour the squash mixture into the dry mixture and stir just until combined.
Scrape the batter into the loaf pan. Bake loaf for 50-60 minutes, until a toothpick inserted in the center of the loaf comes out with no batter on it. Cool on rack for at least 10 minutes before slicing. When completely cool, wrap tightly and store in the refrigerator for up to a week.