Place the pistachios in a food processor and pulse to a bread crumb-like texture. Pour them into a large bowl. Place half the cauliflower florets in the food processor and pulse until they're a couscous-like texture. Add to the pistachios and repeat with the remaining florets.
Add the lemon juice, olive oil, mint, and preserved lemon to the cauliflower mixture and stir to combine. Season with the salt and add more olive oil and lemon juice to taste, if desired. Refrigerate for at least 1 hour before serving to allow the flavors to meld.
Make the cauliflower purée
In a small saucepan, bring the cauliflower, milk, and salt to a boil. Simmer until the cauliflower is very tender, 3 to 5 minutes. Drain the cauliflower, reserving the milk. Place the cauliflower in a blender, add the butter and lemon juice, and process until smooth, adding a little reserved milk if necessary to attain a purée. Season with salt and add more butter to taste, if desired. Cover to keep warm while you cook the fish.
Fry the fish
Pat the fish dry and season on both sides with salt. Heat the glug of olive oil—just enough to coat the surface—in 1 or 2 large skillets over medium heat. Place the fish in the skillet, skin side down (or, if the fish has no skin, the side that had the skin), until crisp and golden, about 2 minutes. Flip the fish and continue to cook until the fish is opaque throughout, another 2 to 3 minutes, depending on the size of the fish.
To assemble, place a dollop cauliflower purée on each plate, top with cauliflower couscous, and place a fish fillet, skin side up, alongside. Drizzle with extra-virgin olive oil as a garnish.
Notes
Couscous Variation
Try adding some soaked golden raisins or chopped Medjool dates and a pinch cumin to the couscous.