2 cups buttercup squash purée (yielded from 2 to 3 pounds buttercup squash)
1 small leek, white and light green parts only, split lengthwise, sliced and washed
½ small onion, sliced
1 clove garlic, sliced
1 tablespoon olive oil
2 cups vegetable stock or water
Preheat oven to 450ºF.
Cut squash in half length-wise and scoop out seeds.
Salt squash and place skin side down on a baking sheet.
Cover the bottom of the baking sheet with water. Cover squash with aluminum foil and bake until tender, about 45 minutes. Let cool. Scoop out the flesh into a strainer. Squeeze out excess moisture in batches (but not completely dry). Measure out 2 cups of squeezed squash and pass through a food mill into a large bowl.
In a 2-quart pot heat olive oil over low heat. Add the remaining vegetables, salt and cook until tender, about 7 minutes. Add the stock, bring to a boil and simmer for 10 minutes. Add squash and carefully purée in a blender or food processor. Adjust seasoning.