Preheat oven to 375 degrees.
Spray baking sheet with nonstick spray, sprinkle halves of squash with salt and pepper. Arrange cut side down on baking sheet. Roast squash until tender, about 40-50 minutes. Scoop squash into medium bowl and set aside.
Heat oil in heavy large pot over medium heat. Add chopped onion and sauté until pale golden, about 10 min.
Mix in chopped celery and garlic, add 1 cup broth. Cover and simmer 10 minutes, stirring occasionally.
Add squash, 5 cups broth and cumin. Cover and simmer 20 minutes to blend flavors.
Puree soup until smooth.
Add black beans, corn, bell pepper, cilantro, thyme and Serrano chili. Cover and simmer 10 minutes.
Season to taste with salt & pepper.