1. Preheat oven to 400 degrees with rack on the top third. Cut squash in half, scoop out seeds, then cut into 2-inch wedges. Combine squash, onion, and garlic on a rimmed baking sheet. Drizzle generously with oil and season with salt; toss to coat, then spread in a single layer. Roast until squash is tender when pierced with the tip of a sharp knife, about 30 minutes, rotating pan and flipping vegetables halfway through (remove garlic and reserve if browning too quickly). Let cool slightly; when cool enough to handle, remove skins from squash and discard.

2. Heat 2 tablespoons oil in a large saucepan over medium-high. Add chile strips and spices; toast, stirring occasionally, until fragrant and chile has softened slightly (1-2 minutes). Add roasted vegetables and broth. Bring to a boil, then reduce heat to low and simmer, stirring occasionally, for 30 minutes.

3. Meanwhile, prepare garnishes: Using tongs, roast corn over an open flame on medium-high, or under the broiler, turning occasionally, until kernels are tender and charred in spots, about 6 minutes. Transfer to a cutting board. When cool enough to handle, cut corn kernels off of cob. Heat pepitas in a small skillet over medium-high, stirring, until toasted and popping (about 4 minutes). Set aside. To make crema, mix sour cream and 2 tablespoons water.

4. In batches, puree soup in a blender. Season with salt. Strain for smoother texture, if desired. Return blended soup to the pot and reheat gently on the stove. Divide into bowls and serve with blackened corn kernels, toasted pepitas, crema, and lime wedges.