DIRECTIONS
Preheat an oven to 350°F (180°C).
Spread the walnuts evenly on a baking sheet. Toast in the oven until golden brown and fragrant, 8 to 10 minutes. Set aside to cool.
Position an oven rack so that it is at least 8 inches (20 cm) from the heating element and preheat the broiler.
Grate the zest from the lemons. In a food processor, combine the lemon zest, olive oil, parsley, garlic and 1/4 tsp. salt and pulse until coarsely chopped. Add the walnuts and pulse until finely chopped. You want the mixture to retain some small walnut pieces, but there should be no large chunks remaining.
Squeeze the juice from 1 of the lemons into a broiler-proof baking dish. Add the butter and thyme to the dish and transfer to the oven until the butter is melted, about 2 minutes. Remove from the oven.
Season the fish on both sides with salt and pepper. Dip each fillet in the lemon-butter mixture to coat both sides, then arrange the fillets in the baking dish. Top each with an even coating of the walnut mixture, completely covering the top of each fillet. Transfer to the oven and broil until the fish just begins to flake when you press on it but is still translucent, 6 to 8 minutes; be careful not to overcook the fish.
Using a spatula, transfer the fillets to individual plates and spoon the lemon-butter mixture from the dish over the top. Cut the remaining lemon into wedges and serve alongside the fish for squeezing on top if desired. Serves 4.