Preheat oven to 425°F and line a baking sheet with parchment paper. Pat cod fillets dry and season lightly.
Combine pistachios, breadcrumbs, Parmesan, parsley, garlic powder, salt, and pepper. Drizzle with olive oil and toss.
Mix eggs with Dijon mustard in a shallow dish.
Dip cod fillets in egg wash, then press firmly into pistachio mixture. Place on baking sheet.
Drizzle fillets with melted butter and lemon juice. Bake 12–15 minutes until cod flakes easily at 145°F.
Let rest 2–3 minutes before serving. Garnish with lemon wedges, pistachios, or herbs.