Heat oil in a large stockpot over medium-high heat. Add onion and jalapeño, and sauté for 5 minutes, stirring occasionally, until the onion is soft and translucent. Add the garlic and sauté for an additional 1-2 minutes until fragrant. Stir in the flour until it is evenly combined and sauté for 1 more minute, stirring occasionally.
Gradually add in the chicken broth, stirring until combined. Then stir in the cooked chicken, green chiles, white beans, corn (if using), and cumin. Continue cooking until the soup reaches a simmer, then reduce heat to medium-low and continue simmering for about 5 minutes. Stir in the warm milk until combined. Then give the soup a taste, and season generously with salt and pepper to taste.
Serve warm in bowls (adding tortilla “bowls” if you’d like), topped with your desired garnishes.