Preheat your oven to 375°F (190°C). While the oven warms up, dice the chicken breast into bite-sized pieces, slice the mushrooms, dice the onion, and mince the garlic.
Heat 2 tablespoons of olive oil in a large skillet over medium heat. Add the diced onion and minced garlic, sautéing until fragrant and translucent, about 3-4 minutes. Add the diced chicken breast and cook until it’s no longer pink on the outside, about 5 minutes. Then, add the sliced mushrooms and cook until they release their moisture and begin to brown, about 4-5 minutes. Season the mixture with salt, pepper, and dried thyme.
In a large mixing bowl, combine the uncooked white rice, chicken broth, and the can of cream of mushroom soup. Stir well to mix all the ingredients evenly.
In the casserole dish, spread the sautéed chicken, mushrooms, and onion mixture evenly. Pour the rice and soup mixture over the top. Then, sprinkle the frozen peas evenly across the surface. Give the casserole a gentle stir to distribute the peas without disturbing the layering too much.
Cover the casserole dish tightly with aluminum foil and bake in the preheated oven for 45 minutes.
After 45 minutes, carefully remove the foil and sprinkle the shredded cheddar cheese evenly on top of the casserole. Return the dish to the oven uncovered and bake for an additional 10-15 minutes until the cheese is melted, bubbly, and slightly golden.
Allow the casserole to rest for 5-10 minutes before serving. This resting time helps the dish set and makes it easier to portion.
