Preheat oven to 375°F (190°C) and grease a 9x13-inch baking dish.
Heat oil in a skillet over medium heat; sauté onion and bell pepper until softened, about 4–5 minutes.
Add garlic, cumin and chili powder; cook 30 seconds until fragrant.
Stir in shredded chicken and diced tomatoes with chiles; simmer 2–3 minutes to combine flavors.
In a bowl, whisk together cream of chicken soup and sour cream until smooth; season with salt and pepper.
Fold the soup mixture into the chicken–tomato mixture until evenly coated.
Arrange a single layer of tortilla wedges in the bottom of the prepared dish.
Spoon one-third of the chicken mixture over the tortillas and sprinkle with one-third of the cheese; repeat two more layers.
Cover the dish with foil and bake for 20 minutes.
Remove foil and bake an additional 5–10 minutes until bubbly and cheese is melted and golden.
Let casserole rest 5–10 minutes before slicing. Garnish with cilantro, diced tomatoes and jalapeños if desired, then serve warm.