Preheat the broiler. Broil the poblanos until blackened, 10 minutes. Transfer to a bowl, cover and let cool. Peel, seed and chop the poblanos.
Preheat the oven to 400 degrees . On a baking sheet, toss the tortilla strips with 1 tablespoon oil and the cumin. Bake until golden, about 10 minutes; season with salt.
Meanwhile, in a large, deep skillet, heat the remaining 2 tablespoons oil over medium-high heat. Add the corn and cook until charred at the edges, 10 to 12 minutes. Add the red onion and garlic, season with salt and pepper and cook until the onion is softened, about 5 minutes. Stir in the broth, tomatoes and poblanos.
Divide the tortilla strips among 4 soup bowls. Add the avocado and a squeeze of lime juice. Ladle the soup over the avocado. Top with cilantro; serve with the sour cream.