Preheat the oven to 375°F / 190°C / Gas Mark 5, no fan (if you have a convection oven, reduce the temperature by about 20° or follow the manufacturer's instructions). Adjust the oven rack to one position above the center of the oven.
Process the pistachios in a food processor until finely chopped (don't process them for too long, they should be still a little chunky and not finely ground), finely chop the shallot and garlic (don't put them in the food processor with the pistachios, process them separately or chop them by hand).
In a small bowl, combine ground pistachios, chopped shallots and garlic, and all the other ingredients (mustard, thyme, lemon zest, butter, breadcrumbs). Season it with salt and pepper and stir until combined.
1/3 cup shelled, 2 tablespoons finely chopped shallots, 1 clove garlic, 1 teaspoon Dijon mustard, 1/2 teaspoon dried thyme, 1 teaspoon grated lemon zest, 2 tablespoons soft butter, 1 tablespoon fine breadcrumbs
Pat dry fish fillets, check them for grates, and season them with salt and pepper on both sides. Place the fillets in a small baking dish and cover with the pistachio paste.
2 fish fillets
Bake the fish for about 13-15 minutes or until a thermometer registers 141°F/61°C (after the resting time it should come to the safe temperature of 145°F/63°C). Leave to rest for 5 minutes then serve.
Enjoy!