Preheat an outdoor grill for medium-high heat and lightly oil the grate.
Grill chicken thighs until browned, about 5 minutes per side. Transfer to a cutting board; dice.
Grill Hatch chile peppers, turning frequently, until blackened and blistered, 2 to 3 minutes. Cool until easily handled, about 5 minutes. Peel off skin and dice flesh into small pieces.
Place diced chile peppers in a large pot. Add 1 tablespoon of water and onion; cook and stir until onion softens, about 5 minutes. Stir in diced chicken, remaining water, garbanzo beans, and chili seasoning mix. Simmer until flavors combine, about 20 minutes.