Rinse salmon, pat dry, remove pin bones (if not already removed) and cut into serving pieces. Place skin side down on parchment paper-lined baking sheet.
Brush with a little oil and sprinkle with salt and pepper.
Whisk together Dijon mustard and lemon juice and spread on top of salmon.
Gently press the chopped pistachios into the mustard spread.
Bake at 375°F. for 10-15 min. (depending on the thickness of the fillets) or until fish flakes easily with a fork but is still a little translucent in the center. Remove from oven and let stand for 5 minutes to finish cooking before serving.
How to tell when salmon is done: It's easy to overcook salmon. Check it at 10 minutes. If it is beginning to flake when poked with a fork, but is still slightly translucent in the center of the fillet, it is probably done. Remember to let it rest for 5 minutes after removing from the oven. It will continue to cook a little on the counter. You can check the internal temperature with an instant-read thermometer. It should be 145°F.