Preheat the oven: Start by preheating your oven to 375°F (190°C).

Season the chicken: Place the chicken breasts on a clean cutting board. Rub them with 1/2 teaspoon salt, 1/4 teaspoon black pepper, minced garlic, and taco seasoning. Make sure the seasoning coats the chicken evenly for maximum flavor.

Prepare the vegetables: Slice the onion and bell peppers into thin strips (julienne-style).

Arrange the casserole: In a casserole dish, drizzle 1 teaspoon of olive oil (or use a non-stick spray if you prefer to reduce fat). Layer the seasoned chicken breasts on the bottom of the dish. Then, arrange the sliced onions and bell peppers on top of the chicken.

Cover and bake: Cover the dish with aluminum foil to lock in the steam, which helps the chicken and veggies cook evenly. Place it in the preheated oven and bake for 25 minutes.

Uncover and add cheese: After 25 minutes, remove the casserole from the oven. Carefully remove the aluminum foil and sprinkle 1/2 cup shredded mozzarella cheese evenly over the top of the vegetables. Return the dish to the oven, uncovered, for an additional 10 minutes, or until the cheese is melted and bubbly.

Check for doneness: Make sure the chicken is fully cooked by checking its internal temperature. It should reach 165°F (74°C). If you don’t have a thermometer, slice into one of the chicken breasts to check if the center is no longer pink.

Serve: Once the casserole is done, let it rest for 5 minutes. This helps the juices redistribute throughout the chicken, making it tender and juicy. Serve 1/4 of the casserole per person, and enjoy your flavorful Bell Pepper Chicken Casserole!