Preheat your oven to 400°F and line a baking sheet with parchment paper for easy cleanup.
Slice the carrots and celery into 1-inch thick coins and half-moons.
Toss the carrots and celery on the baking sheet with half of the olive oil, sea salt, and black pepper.
Roast the vegetables for 20-22 minutes until they are tender and slightly caramelized at the edges.
While the vegetables roast, cut the chicken breast into 1-inch cubes and thinly slice the cremini mushrooms.
Heat the remaining olive oil in a large skillet over medium-high heat.
Add the chicken to the skillet and cook for 5-7 minutes until golden brown and fully cooked through.
Stir in the sliced mushrooms, minced garlic, and dried thyme, sautéing for 4-5 minutes until the mushrooms have released their moisture and turned golden.
Remove the roasted carrots and celery from the oven and fold them into the skillet with the chicken and mushrooms.
Toss everything together for one minute to meld the flavors and serve immediately.