Step 1: Prepare Your Oven and Baking Sheet
First things first, preheat your oven to 400°F (200°C). Position a rack in the middle of the oven to ensure even cooking. Line a large, rimmed baking sheet with parchment paper or aluminum foil. This is a crucial step for easy cleanup and prevents the salmon from sticking, especially if you are using skinless fillets.

Step 2: Prepare the Pistachio Crust
The texture of your crust is paramount. You are aiming for a coarse, crumbly texture, not a fine powder. A fine powder can become pasty when mixed with the other ingredients.

• Using a Food Processor: Place the shelled pistachios in the bowl of a small food processor. Pulse 5-7 times in short bursts. You want a mix of small pieces and some slightly larger ones for texture. Be careful not to over-process.
• By Hand: If you don’t have a food processor, you can achieve the same result manually. Place the pistachios in a sturdy zip-top bag, seal it while pressing out most of the air, and lay it flat on a cutting board. Use a rolling pin, meat mallet, or even the bottom of a heavy pan to gently crush the nuts until you reach the desired consistency.
  Once crushed, transfer the pistachios to a shallow dish or plate.

Step 3: Prepare the Salmon Fillets
Remove the salmon from the refrigerator about 15 minutes before cooking to allow it to come closer to room temperature, which promotes more even cooking. Open the package and pat the salmon fillets completely dry on all sides with a paper towel. This is one of the most important steps. A dry surface allows the mustard-honey glaze to adhere properly and, if using skin-on salmon, helps the skin get crispy. Place the salmon fillets on the prepared baking sheet, skin-side down if applicable. Lightly season the tops with a pinch of salt and freshly ground black pepper.

Step 4: Create and Apply the Glaze
In a small bowl, whisk together the Dijon mustard, honey, and olive oil until you have a smooth, emulsified mixture. This is the “glue” that will hold the delicious pistachio crust to the salmon while also infusing it with flavor. Using a pastry brush or the back of a spoon, spread this mixture evenly over the top and sides of each salmon fillet. Be generous; you want a nice, thick layer to ensure the crust has something substantial to stick to.

Step 5: Apply the Pistachio Crust
Now for the fun part. Sprinkle the crushed pistachios evenly over the top of each glazed fillet. Gently press the nuts down with your fingers or the back of a spoon to help them adhere securely to the mustard-honey mixture. Ensure each fillet is generously coated. Any nuts that fall onto the baking sheet will toast nicely in the oven and can be scooped up and served alongside the fish.

Step 6: Bake to Perfection
Place the baking sheet on the middle rack of your preheated oven. Bake for 12-15 minutes. The exact cooking time will depend on the thickness of your fillets. The salmon is done when it is opaque and flakes easily when gently prodded with a fork. For the most accurate result, use an instant-read thermometer inserted into the thickest part of the fillet; you are looking for a temperature of 130-135°F (54-57°C) for a medium-well finish. The salmon will continue to cook slightly from residual heat after you remove it from the oven. The pistachio crust should be lightly golden and fragrant.

Step 7: Rest and Serve
Remove the salmon from the oven and let it rest on the baking sheet for 3-5 minutes before serving. This allows the juices to redistribute throughout the fillet, resulting in a moister, more flavorful piece of fish. Serve immediately.