Preheat your oven to 400°F and line a baking sheet with parchment paper. This helps the fish cook evenly while preventing the crust from sticking. I like using parchment because it also keeps cleanup very quick and easy.
In a mixing bowl, combine the chopped pistachios, panko breadcrumbs, Parmesan cheese, garlic powder, paprika, salt, and pepper. Stir everything well so the flavors blend evenly. The pistachios create a crunchy texture that tastes amazing once baked.
Pat the halibut fillets dry using paper towels to remove excess moisture. This step helps the crust stick properly and gives the fish a better texture. Brush the top of each fillet lightly with Dijon mustard.
Press each halibut fillet into the pistachio mixture, coating the top evenly. Gently press the crust down so it sticks firmly during baking. Place the coated fillets onto the prepared baking sheet with space between each piece.
Bake the fish for about 15 to 18 minutes until the crust becomes golden brown. The fish should flake easily with a fork when fully cooked. Avoid overbaking because halibut can dry out quickly if left in too long.
While the fish bakes, melt the butter in a small saucepan over low heat. Stir in the lemon juice and lemon zest until smooth and fragrant. The sauce should stay silky and light without boiling too much.
Remove the baked halibut from the oven and drizzle the warm lemon butter generously over the top. Sprinkle chopped parsley for freshness and color. Serve immediately while the crust is still crispy and delicious.