Preheat the oven to 400 degrees F. Line a baking sheet with parchment paper.
Using a small knife, remove the peel, pith and outer membrane of the grapefruit, tangerine and orange. Working over a bowl, slice each into segments; remove the seeds. Drizzle with olive oil and season with salt and pepper. Cover and refrigerate until ready to serve.
In a food processor, pulse the pistachios and garlic salt until the mixture resembles small pebbles. Transfer to a shallow dish.
Season both sides of the fish fillets with salt and pepper. Use a pastry brush to coat the top side of each piece of fish with the Dijon mustard. Gently press the mustard-coated side of the fillets into the pistachio mixture. Transfer them pistachio crusted-side up onto the prepared baking sheet.
Bake until the fish is opaque and flakes easily with a fork, 10 to 12 minutes, depending on the thickness of the fillets. Transfer to a serving plate and top with the citrus salsa. Drizzle with olive oil and season with more salt and pepper before serving.