Preheat oven to 350 degrees F (175 degrees C).

Line a baking sheet with parchment paper or aluminum foil.

Place halved and seeded butternut squash, quartered onions, and foil-wrapped garlic on the baking sheet.

Roast in the center of the oven for 45 to 60 minutes, until the squash is tender.

Remove from oven and let cool.

Squeeze roasted garlic cloves out of their skins into a food processor.

Scrape the flesh from the roasted squash and add it to the food processor with the roasted onion.

Puree until smooth, adding vegetable broth if necessary to achieve desired consistency.

Transfer the pureed mixture to a stockpot.

Stir in vegetable broth, bay leaf, brown sugar, curry powder, oregano, cinnamon, nutmeg, salt, and pepper.

Bring to a boil and then reduce heat to low and simmer gently for 10 minutes.

Remove from heat and stir in plain yogurt.

Remove bay leaf and serve hot.

Garnish with fresh parsley if desired.