Heat broiler and line a baking sheet with aluminum foil. Toss potato, onion, corn, carrot, fennel, garlic, poblano, and oil in a large bowl. Arrange vegetables in a single layer on prepared sheet and broil, tossing once halfway through, until browned, about 12 minutes.
Transfer vegetables to a large pot and stir in spices, salt, wine, and 3 quarts water. Bring to a simmer over medium-high heat, then reduce heat and simmer 45 minutes.
Remove soup from heat and whisk in coconut milk. Garnish with cilantro and scallions before serving.