4 tablespoons unsalted butter
1 medium red onion, cut into 1/4-inch dice
4 garlic cloves, minced
1 serrano chile, seeded and minced
1 1/2 teaspoons ground cumin
1/2 teaspoon cayenne pepper
2 cups yellow split peas, soaked in water for 1 hour and drained
8 1/2 cups water
One 15-ounce can unsweetened pumpkin puree
3/4 pound fresh sugar pumpkin or butternut squash, peeled and cut into 1/4-inch dice
1 1/2 tablespoons fresh lemon juice
salt and freshly ground pepper

1. In a large pot, melt the butter. Add the onion, garlic and chile and cook over moderately high heat until the onion is softened, 4 minutes. Add the cumin and cayenne and cook until fragrant, about 1 minute. Add the split peas and the water, then whisk in the pumpkin puree and bring to a simmer. Cover and cook over moderately low heat, stirring occasionally, until the split peas are tender, about 2 hours.

2. Stir the diced pumpkin into the soup and simmer over moderately low heat until tender, about 30 minutes. Remove from the heat. Stir in the lemon juice, season with salt and pepper and serve.