Preheat the oven to 350 degrees. Remove the chicken from the refrigerator and bring to room temperature. Bring a medium pot of water with a large pinch of salt to a boil. Thinly slice the mushrooms. Mince or grate the garlic. Strip the thyme leaves off the sprig and roughly chop.

Cook the rice: Add the rice to the boiling water and boil for about 30 minutes, until tender. Drain.

Meanwhile, heat a large drizzle of oil in a large pan over medium heat. Pat dry the chicken with a paper towel, then season on all sides with salt and pepper. Add the chicken to the pan and cook for 2-3 minutes per side, until browned, but not yet cooked through. Place the chicken on a baking sheet and transfer to the oven for about 10-12 minutes, until just cooked through.

Heat another large drizzle of oil in the same pan over medium heat. Add the thyme and garlic to the pan and cook for 30 seconds, until fragrant. Add the mushrooms to the pan and cook, tossing for 5-6 minutes, until browned. Season with salt and pepper.

Add 1 cup water and the stock concentrates to the pan. Scrape up any browned bits from the bottom of the pan. Nestle the chicken in the sauce while it reduces. Bring to a simmer for 3-4 minutes, or until reduced by half. Remove the pan from heat and stir in the sour cream and 2 teaspoons Dijon mustard. Season to taste with salt and pepper.

Finish and plate: Thinly slice the chicken and serve on a bed of brown rice. Drizzle with the Dijon mushroom sauce and enjoy!