Heat olive oil in a a large stockpot or dutch oven over medium heat. Add onion and jalapeño and sauté until tender, about 5-8 minutes. Add in garlic and cumin and sauté until fragrant, about one minute.
Add shredded chicken, green chiles, beans, chicken stock, oregano, salt, and pepper to the pot. Bring the mixture to a boil, reduce the heat, and let it simmer for about 20 minutes.
In a separate small bowl, mix together the cornmeal and milk. Stir mixture into chili. Let simmer for another 10 minutes to thicken.
Serve chili topped with shredded cheddar cheese and sliced avocado.