Adjust rack to top position (top and middle positions for 4 servings) and preheat oven to 450 degrees. Wash and dry produce.

Cut potatoes into ½-inch-thick wedges. Trim and halve zucchini lengthwise; cut crosswise into ½-inch-thick half-moons. Zest and quarter lemon. Mince chives.

Toss potatoes on one side of a baking sheet with a large drizzle of olive oil, salt, and pepper. (For 4 servings, spread potatoes out across entire sheet.)

Roast on top rack for 10 minutes (you’ll add the zucchini then).

Meanwhile, in a large bowl, toss zucchini with a drizzle of olive oil and a pinch of salt and pepper.

Once potatoes have roasted 10 minutes, remove sheet from oven; add zucchini to empty side. (For 4 servings, leave potatoes roasting and add zucchini to a second sheet; roast on middle rack.)

Return to top rack until potatoes are golden brown and zucchini is lightly browned, 12-15 minutes more. Carefully toss zucchini with lemon zest.

While potatoes and zucchini roast, pat salmon* dry with paper towels; season all over with salt and pepper.

Heat 1 TBSP butter and 1 TBSP olive oil in a large pan over medium-high heat. Add salmon, skin sides down, and cook until skin is crisp and fish is almost cooked through, 5-7 minutes. Flip and cook until cooked through, 1-2 minutes more.

Turn off heat; remove from pan and set aside. Wipe out pan.

Return same pan to medium-high heat. Add stock concentrate, half the mustard (all for 4 servings), juice from half the lemon, and ¼ cup water (⅓ cup for 4). Simmer until slightly thickened, 2-3 minutes. Turn off heat.

Stir in sour cream, half the chives, and 1 TBSP butter (2 TBSP for 4) until combined. (TIP: If sauce seems too thick, add more water 1 tsp at a time until it reaches a drizzling consistency.) Season with salt and pepper.

Divide salmon, potatoes, and zucchini between plates. Drizzle sauce over salmon. Garnish with remaining chives if desired. Serve with remaining lemon wedges on the side.