Preheat oven to 375 degrees. Place tomatillos on a small rimmed baking sheet. Roast in oven, turning once midway through, until they are softened and slightly charred, about 25 minutes. Remove from oven, and let cool slightly. Transfer to the bowl of a food processor; add cilantro, 1 garlic clove, and lime juice. Process until smooth and combined; set aside.
Lightly coat bottom of a large nonstick saucepan with cooking spray. Add onion, remaining garlic clove, and diced jalapeno; cook, stirring occasionally, over medium heat until onion is softened, about 7 minutes. Add cumin, tomatoes, corn, and chicken stock. Bring liquid to a boil; reduce heat, and simmer until vegetables are tender, 12 to 15 minutes.
Remove from heat; stir in 3 tablespoons reserved tomatillo mixture, and season with pepper. Add sliced avocado. Ladle into serving bowls; garnish with jalapeno slices, if desired. Serve with remaining tomatillo pesto.