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

Dice potatoes into ½-inch pieces. Trim and halve Brussels sprouts. Quarter lemon.

Place potatoes in a medium pot with enough salted water to cover by 2 inches. Bring to a boil and cook until tender, 10-12 minutes. Reserve ½ cup potato cooking liquid, then drain; return potatoes to pot.

Keep covered off heat until ready to mash in Step 5.

While potatoes cook, toss Brussels sprouts on a baking sheet with a drizzle of oil, salt, and pepper. Roast on top rack for 10 minutes (you'll add the salmon then).

Meanwhile, place 2 TBSP butter (3 TBSP for 4 servings) in a medium microwave-safe bowl. Microwave until melted, 30-60 seconds. Let cool slightly, then stir in pecans, panko, and a pinch of salt and pepper.

Pat salmon* dry with paper towels. Rub a drizzle of oil on skin sides; season all over with salt and pepper.

Spread Dijon mustard on flesh sides of salmon; mound with panko mixture, pressing firmly to adhere.

Once Brussels sprouts have roasted 10 minutes, remove baking sheet from oven and carefully push to one side. (For 4 servings, leave Brussels sprouts roasting; add salmon to a second sheet and roast on middle rack, swapping rack positions halfway through.)

Place salmon, skin sides down, on opposite side of sheet. Roast on top rack until salmon is cooked through and Brussels sprouts are browned and tender, 8-10 minutes.

Meanwhile, add sour cream and 1 TBSP butter (2 TBSP for 4 servings) to pot with potatoes. Mash until smooth and creamy, adding splashes of reserved potato cooking liquid as needed. Season to taste with salt and pepper.

Divide Brussels sprouts, mashed potatoes, and salmon between plates. Drizzle smoky mustard over salmon and serve with lemon wedges on the side.