Heat the oven to 375°F/190°C. Line a sheet pan with parchment or lightly oil it.
Combine the pistachios and walnuts in a food processor and pulse to a coarse crumb, or chop finely by hand.
Pat the salmon dry and arrange on the prepared pan. Brush with Dijon mustard and season with garlic powder, salt, and pepper. Press the nut mixture evenly over the top of each fillet.
Stir the date syrup and orange zest in a small bowl. Drizzle over the salmon, then lightly coat with olive oil.
Roast until the salmon is just cooked through and flakes easily, 10 to 12 minutes for thinner fillets or 15 to 18 minutes for thicker pieces. Broil for 1 minute to deepen color, if desired.
Finish with a squeeze of lemon juice, additional orange zest, and a light drizzle of date syrup. Serve immediately.