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Adjust rack to top position (top and middle positions for 4 servings) and preheat oven to 425 degrees. Wash and dry all produce.
Dice potatoes into ½-inch pieces (peel first if desired). Finely chop walnuts. Trim and quarter zucchini lengthwise; slice crosswise into ½-inch-thick pieces. Halve, peel, and dice onion. Zest and quarter lemon.
Place 2 TBSP butter (3 TBSP for 4) in a medium microwave-safe bowl. Microwave until melted, 30 seconds. Let cool slightly, then stir in walnuts, panko, and a pinch of salt and pepper.