Preheat oven to 350 degrees. Place the two halves of the spaghetti squash face down into a baking pan with a bit of water — just barely covering the bottom of the pan. Cover tightly with foil and bake for 40 minutes. Check for readiness by trying to lift off a few strands of squash with a fork. When ready, the squash will look like spaghetti and come up easily.

In a large pot, sauté onions in oil over low heat until golden and limp, around 30 minutes. (Caramelizing onions does take a while, but the flavor is worth it, so leave yourself time for this step.) Add remaining ingredients to pot and bring to a boil. Lower heat and simmer soup for 30 minutes.

Meanwhile, scrape the squash into a large bowl and remove one cup of it to a separate bowl. Transfer the larger amount of squash to the soup and continue to cook until vegetables are soft when pierced with a fork. Remove from heat.

Remove carrot from soup and mash with a fork. Using an immersion blender, blend soup until smooth. Return mashed carrot and remaining one cup spaghetti squash to soup and stir well to combine. Adjust seasonings to taste.