Preheat oven to 400°F; spray 2 18X12X1 inch baking sheets with nonstick spray. Toss carrots, onions, and oil in large bowl to coat. Arrange squash halves, cut side up, on baking sheets; divide butter among squash cavities. Scatter carrots and onions around squash. Sprinkle with salt and pepper.
Roast vegetables until tender and golder, stirring carrots and onions occasionally, about 1 hour and 30 minutes. Cool vegetables 15 minutes.
Working in batches, scoop roasted squash from skins into processor. Add some carrots and onions. Puree until almost smooth (some texture should remain. Transfer puree to heavy large pot after processing each batch. Add 10 cups broth to puree in pot and bring to simmer. Cook 10 minutes to blend flavors. Add more broth by cupfuls to thin, if desired. Season to taste with salt and pepper. (Can be made 2 days ahead. Cool slightly. Chill uncovered until cold, then cover and keep chilled. Rewarm over medium heat, thinning with more broth if desired.) Ladle into soup bowls; sprinkle with parsley.