Preheat oven to 400F and line a medium sheet pan with parchment paper. Combine halved tomatillos, whole garlic cloves, chili powder, cumin, paprika, and 1 tablespoon oil on pan and toss until evenly combined. Arrange in a single layer and season with salt and pepper to taste.
Roast tomatillos at 400F 20-25 minutes until tender and juicy. Allow tomatillos to cool slightly then finely chop. Remove skins from garlic and finely mince.
In a medium pot or Dutch oven, heat remaining 1 tablespoon vegetable oil over medium-high heat. Add onion and cook 3-5 minutes until tender.
Add tomatillos, garlic, chicken, beans, sweet corn, and chicken broth to pot and bring to a simmer over medium-high heat. Reduce heat to medium-low, cover pot, and allow to simmer 10-15 minutes, stirring occasionally, to allow the flavors to meld.
In a small bowl, whisk milk and flour until smooth. Remove chili from heat and stir in milk-flour mixture and sour cream. Season chili with salt and pepper to taste and serve warm with optional toppings. Enjoy!