Heat a large pot over medium heat. Add the butter and let it melt, then add the diced onion and bell peppers. Sauté for about 5 minutes, or until the vegetables have softened.
Stir in the garlic and cook for another minute, until fragrant.
Add the seasonings and stir to coat the vegetables. Then add the potatoes, corn, chicken stock, milk, and raw chicken, making sure the chicken is fully submerged in the liquid.
Bring the soup to a boil, then cover and reduce the heat to a gentle simmer. Cook for about 30 minutes, or until the chicken is cooked through and the potatoes are tender.
Remove the chicken from the pot and shred it using two forks. Set it aside while you finish the soup.
In a small bowl, whisk together the cornstarch and cold water until smooth. Stir the slurry into the soup, bring it back to a boil, and let it cook for 1 minute to thicken.
Use an immersion blender or blender to blend about half of the soup, leaving plenty of chunks for texture. If you don't have an immersion blender, carefully blend about half of the soup in a regular blender, then return it to the pot.
Stir the shredded chicken back into the soup and let everything heat through for another minute or two.
Serve warm with your favorite toppings. I love adding shredded cheddar cheese, bacon bits, and sliced green onions.