Step 1

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

Step 2

Heat a large drizzle of olive oil in a large pan over medium-high heat. Season chicken all over with salt and pepper. Cook in pan 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.

Step 3

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

Step 4

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.

Step 5

Stir stock concentrate, ½ cup water, and thyme sprigs into pan while scraping up any browned bits on bottom. Reduce heat to low and simmer until reduced by half, 2-3 minutes. Remove pan from heat and let cool 1-2 minutes, then stir in mustard and sour cream. Season with salt and pepper. Discard thyme sprigs. Meanwhile, halve lemon; cut one half into wedges.

Step 6

Toss arugula with 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 arrange chicken on top. Drizzle with mushroom sauce and sprinkle with reserved thyme leaves (to taste). Serve with salad and lemon wedges.