Adjust rack to top position (top and middle positions for 4 servings) and preheat oven to 450 degrees. Wash and dry all produce. Cut potatoes into 1/2-inch-thick wedges. Trim and halve zucchini lengthwise; cut crosswise into 1/2-inch-thick half-moons. Mince chives. Zest and quarter lemon (quarter both lemons for 4).

Toss potatoes on one side of a baking sheet with a large drizzle of olive oil, salt, and pepper. (For 4 servings, spread out across sheet.) Roast on top rack for 10 minutes (we’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 from oven; add zucchini to empty side of sheet. (For 4, leave potatoes roasting and add zucchini to a second baking sheet; roast on middle rack.) Return to oven until potatoes are golden brown and zucchini is lightly browned, 12- 15 minutes more. Once zucchini is done, remove from oven; toss with lemon zest.

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 browned and slightly crisp, 3-4 minutes. Flip and cook until fish is opaque and cooked through, 3-4 minutes more. Turn off heat; transfer to a paper-towel-lined plate. Wipe out pan.

Return same pan to medium-high heat. Add 1/4 cup water (1/3 cup for 4 servings), juice from half the lemon, stock concentrate, and half the mustard (all for 4). Simmer until slightly thickened, 1-2 minutes. Turn off heat. Stir in half the chives, 1 TBSP butter (2 TBSP for 4), and sour cream. (TIP: If sauce seems too thick, add 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.