How to Make It
Light a grill. In a medium skillet, heat 1 tablespoon of the olive oil. Add the sliced onion and cook over moderate heat, stirring occasionally, until golden brown, about 8 minutes.
In a food processor, puree the parsley leaves with 1/4 cup of the olive oil and scrape the puree into a medium bowl. Add the walnuts and onion to the food processor bowl and process to a paste. With the machine on, slowly add the remaining 1/4 cup of olive oil. Stir the paste into the parsley puree and season with salt and pepper.
Brush the dorado fillets with olive oil and season with salt and pepper. Grill the fish over a hot fire, skin side down, until the skin is crisp, about 2 minutes. Turn the fillets and grill until just cooked through, about 3 minutes longer. Transfer the fillets to plates. Spread the walnut-parsley pesto over the fish. Top with the olives and preserved lemon and serve.