Bring a large pot of water to a boil. Cook chicken immersed in water for 15-20 min. Let cool then pull apart with two forks.
Meanwhile, in a large saucepan, mix together all ingredients for enchilada sauce and bring to a boil. Simmer sauce and cook for 10-15 minutes.
Preheat olive oil over med-high heat. Add garlic, sauteeing for 30 seconds. Add chicken, chili powder, cumin and coriander, tossing to coat. Add chicken broth and salsa, bringing to a boil and simmering for 10 minutes. Stir in cilantro and remove from heat.
Preheat oven to 350 F. Grease a large 9x13 casserole dish with olive oil or butter. Add 5-6 corn tortillas on bottom. Top with pulled chicken, onions, bell peppers and corn. Top with another 5-6 corn tortillas then repeat process until the casserole dish is filled.
Pour enchilada sauce over the top of the casserole, using as much as the sauce as needed (I used all of it but you may not want your enchiladas so saucy!) Bake uncovered in the oven for 15-20 minutes. Top with additional toppings then serve!