Preheat your oven to 375°F (190°C).
Heat the olive oil in a large oven-safe skillet or Dutch oven over medium heat.
Add the diced yellow onion and sliced cremini mushrooms, sautéing for 5-7 minutes until the mushrooms have released their moisture and turned golden brown.
Stir in the minced garlic and diced chicken breast, cooking just until the chicken is opaque on the outside.
Add the dry brown rice, sea salt, black pepper, and dried thyme, stirring for 1 minute to lightly toast the rice.
Pour in the chicken bone broth and bring the mixture to a gentle simmer.
Cover the skillet tightly with a lid or foil and transfer it to the oven.
Bake for 45-50 minutes, or until the rice is tender and the liquid is fully absorbed.
Remove from the oven and let the casserole rest, covered, for 5 minutes to allow the steam to finish the rice texture before fluffing with a fork and serving.