Preheat oven to 350° Fahrenheit. Lightly grease 9x13 baking dish and set aside.
Heat neutral oil in large skillet over medium-high heat. Once oil is hot and shimmering, add onion and bell pepper to skillet. Stir to incorporate, then let veggies cook 2 to 3 minutes or until slightly softened.
Remove skillet from heat. Pour in cream of chicken soup, cream of mushroom soup, and diced tomatoes. Stir to incorporate, then add chili powder, dried oregano, and shredded chicken. Stir until all ingredients are well combined, then set aside.
Line bottom of baking dish with triangles from 6 corn tortillas. Top tortillas with 50% of chicken mixture and spread mixture into one even layer. Top with 1 cup shredded cheese. Repeat layers until all ingredients have been added to baking dish.
Place baking dish in preheated oven. Bake casserole uncovered 35 to 40 minutes, or until cheese is browned and bubbling. Remove baking dish from oven and let casserole cool slightly, then divide into portions and serve.
Recipe Notes
Leftovers: Refrigerate leftover casserole in an airtight container up to 4 days. Reheat in microwave or bake at 350°F until warmed through.
Make it Gluten Free: Use gluten-free soups and make sure your shredded cheese is free from any gluten-containing starches or fillers.