Adjust rack to top position (top and middle positions for 4 servings) and preheat oven to 425 degrees. Wash and dry produce.

Cut potatoes into ½-inch-thick wedges. Trim and thinly slice mushrooms (skip if your mushrooms are pre-sliced!). Halve, peel, and mince shallot. Roughly chop parsley.

Once chicken has roasted 15 minutes, remove baking sheet from oven and carefully add potatoes to empty side of sheet. (For 4 servings, leave chicken roasting and add potatoes to a second baking sheet.)

Return to top rack and roast until chicken is browned and cooked through and potatoes are tender, 20-25 minutes. (For 4, roast potatoes on middle rack, swapping rack positions with chicken halfway through.) TIP: To ensure chicken is cooked through, insert a thermometer into the innermost part of the thigh and breast (avoiding any bones). Once chicken is cooked, the bones may still appear pink—this is OK!

While chicken and potatoes roast, heat a drizzle of olive oil and 1 TBSP butter (2 TBSP for 4 servings) in a large pan over medium-high heat. Add mushrooms and cook, stirring occasionally, until golden brown and slightly crisp, 5-7 minutes. Season with salt and pepper.

Turn off heat; transfer mushrooms to a small bowl. Wipe out pan.

Heat a drizzle of olive oil in same pan over medium-high heat. Add shallot and cook until lightly browned, 1-2 minutes.

Stir in ⅓ cup water, cream sauce base, crème fraîche, stock concentrate, half the vinegar, and remaining garlic powder (⅔ cup water and all the vinegar for 4 servings). Bring to a simmer, then stir in mushrooms, mustard, and half the parsley; cook until slightly thickened, about 1 minute.

Remove from heat; season with salt and pepper.

Once chicken and potatoes are done roasting, let chicken rest for at least 5 minutes.

Transfer chicken and potatoes to a serving platter. Spoon sauce over chicken; garnish with remaining parsley. Serve family style.