Cook the potatoes in salted boiling water for about 30 minutes, or until very tender. Using a masher, purée the potatoes. Add the milk and butter. Season with salt and pepper. Set aside.
Preheat oven to 200°C (400°F).
In a bowl, coat the chicken with the garlic and spices. Season with salt and pepper. In a skillet, heat the oil and brown the chicken. Continue to cook over medium heat for approximately 20 minutes, turning over occasionally, until fully cooked. Remove from heat and shred using a fork and knife.
To assemble, spread the chicken at the bottom of a baking dish. Next, add a layer of salsa, corn, and red beans. Top with the mashed potatoes and sprinkle with cheddar.
Cook in the oven for approximately 30 minutes, or until the top is golden.