Preheat the oven to 400 degrees F (200 degrees C). Line a baking sheet with parchment paper.
Stir mustard, maple syrup, salt, and pepper together in a bowl. Place salmon fillets skin-side down onto the prepared baking sheet and spread mustard mixture on top.
Bake in the preheated oven for about 10 minutes. Sprinkle pistachios on top of the salmon. Bake until salmon flakes easily with a fork, about 10 more minutes.
