Wash and dry all produce. Preheat oven to 350 degrees. Bring a medium pot of salted water to a boil. Trim, then thinly slice mushrooms. Thinly slice garlic. Pick thyme leaves from stems until you have 1 tsp (save remaining sprigs for sauce).

Heat a large drizzle of olive oil in a large pan over medium-high heat. Season chicken all over with salt and pepper. Cook until browned on surface, 2-3 minutes per side. Transfer to a baking sheet. Bake in oven until no longer pink in center, 10-11 minutes. Let rest 5 minutes after removing from oven.

Once water is boiling, add couscous to pot. Cook until al dente, 8-9 minutes, then drain.

Heat a drizzle of olive oil in pan used for chicken over medium heat. Add mushrooms and cook, tossing, until tender and browned, 3-4 minutes. Add garlic and cook until fragrant, about 30 seconds. Season with salt and pepper.

Stir 1/2 cup water, thyme sprigs, and stock concentrate into pan while scraping up any browned bits. Reduce heat to low and simmer until reduced by half, 2-3 minutes. Remove pan from heat. Set aside 1-2 minutes to cool slightly, then stir in sour cream and 1 tsp mustard (we sent more). Season with salt and pepper. Remove and discard thyme sprigs.

Cut lemon into halves. Toss arugula, a squeeze of lemon, and a large drizzle of olive oil in a medium bowl. Season with salt and pepper. Divide couscous between plates and top with chicken. Drizzle with mushroom sauce, then sprinkle with reserved thyme (to taste). Cut any remaining lemon into wedges and serve on the side.