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Adjust rack to top position (top and middle positions for 4 servings) and preheat oven to 425 degrees. Bring 2 TBSP butter (4 TBSP for 4) to room temperature. Wash and dry produce.
Zest and halve lemon.
Using a heavy-bottomed pan or rolling pin, crush walnuts in their bag until finely ground. In a small bowl, combine walnuts, panko, a large drizzle of olive oil, a pinch of lemon zest, and a big pinch of salt and pepper.