Adjust rack to top position and preheat oven to 450 degrees. Wash and dry produce (except green beans). • Dice potatoes into 1⁄2-inch pieces. • Place potatoes in a medium pot with enough salted water to cover by 2 inches. Bring to a boil; cook until tender, 15-20 minutes. Reserve 1⁄2 cup potato cooking liquid (3⁄4 cup for 4 servings), then drain; return potatoes to pot.
Meanwhile, quarter lemon. Halve, peel, and thinly slice shallot. Place shallot in a small bowl; stir in a squeeze of lemon juice and a pinch of salt. Halve tomatoes lengthwise.
In a second small bowl, combine two packets sour cream and 1 tsp Old Bay Seasoning (four packets sour cream and 2 tsp Old Bay for 4 servings). Add water 1 tsp at a time until sauce reaches a drizzling consistency.
Place 1 TBSP butter (2 TBSP for 4 servings) in a medium microwave- safe bowl. Microwave until melted, 30 seconds. Let cool slightly; stir in panko and 1 tsp Old Bay Seasoning (2 tsp for 4). • Pat salmon* dry with paper towels; place, skin sides down, on a lightly oiled baking sheet. (TIP: For easy cleanup, line baking sheet with aluminum foil first.) Brush tops with a thin layer of sauce (save the rest for serving). Mound with panko mixture, pressing firmly to adhere (no need to coat the undersides). Wash and dry bowl. • Roast on top rack until crust is golden brown and salmon is cooked through, 8-10 minutes.
While salmon roasts, pierce green bean bag with a fork; place bag on a microwave-safe plate. Microwave until tender, 1-2 minutes. Carefully open bag; rinse green beans with cold water until cooled. Pat dry with paper towels; trim if necessary. • In bowl used for panko, whisk together mustard, 1 TBSP olive oil (2 TBSP for 4 servings), a big squeeze of lemon juice, and a pinch of sugar until smooth. Season with salt and pepper. • Add green beans, tomatoes, and as much shallot as you like (draining first) to bowl with dressing. Toss to coat; add another squeeze of lemon juice to taste. Season with salt and pepper if desired.
Mash drained potatoes with remaining sour cream and 1 TBSP butter (2 TBSP for 4 servings) until smooth and creamy, adding splashes of reserved potato cooking liquid as needed. Season generously with salt and pepper. • Divide salmon, green bean salad, and mashed potatoes between plates. Drizzle sauce over salmon. Serve with any remaining lemon wedges on the sid