Preheat the oven to 400 degrees F (200 degrees C).
Place tomatillos and poblanos into a roasting pan and drizzle with olive oil, tossing to coat.
Roast in the preheated oven until poblanos are blackened, about 45 minutes. Remove from the oven and let cool enough to handle, about 10 minutes.
Remove skins from poblanos and place them into an electric blender with tomatillos. Add water and blend until smooth.
Heat 1 tablespoon oil in a skillet over medium-high heat. Saute chicken and onion in the hot oil until onions soften and show color and chicken is browned, 5 to 7 minutes. Add blended poblanos and tomatillos, chicken broth, serrano peppers, garlic, cumin, chipotle powder, and salt.
Bring mixture to a boil, reduce heat to medium-low, and let simmer until chicken is no longer pink in the center and flavors have melded, about 1 hour. Stir in cornmeal to thicken.