Make the sauce:
Put the peppers, vinegar, mustard, and garlic in a blender and begin to puree. With the blender running, add the oil in a thin, steady stream. Transfer to a small bowl, stir in the capers, parsley, 1/2 tsp. salt, and 1/8 tsp. pepper. Season to taste with more salt and pepper if needed.
Prepare the salmon:
Soak a cedar plank in cold water for at least 1 hour and up to 12 hours. In a small bowl, mix the sugar, cumin, pimentón, 1-1/2 tsp. salt, and 1/2 tsp. pepper. Sprinkle the mixture over the salmon and let sit at room temperature for 30 minutes, or cover and refrigerate for up to 12 hours.
Prepare a two-zone fire on a gas or charcoal grill: On a gas grill, set two of the burners to high (one if your grill only has two burners) and leave one burner off. On a charcoal grill, push all of the lit coals to one side of the grill.
Set the cedar plank on the grill over the hot zone, cover the grill, and wait until the plank starts to smoke, 2 to 3 minutes. Meanwhile, rub the skinned sides of the salmon fillets with the oil. Slide the plank to the cooler zone and arrange the salmon on top, oiled side down and thicker ends closest to the hot zone. Cover the grill (vents open on a charcoal grill) and cook until the fish is almost cooked through with just a touch of pink in the center—make a nick with a paring knife in the thicker part of a fillet to check—or an instant-read thermometer inserted in a thick part registers 135°F, 10 to 14 minutes. Transfer the fish (still on the plank) to a serving platter and serve with the sauce.
Make Ahead Tips
The sauce can be made up to two days ahead and stored in the refrigerator.
Serve with Sauteed Spinach with Pine Nuts and Raisins. Or turn it into a party with simple tapas like Manchego in Olive Oil.
Photo: Scott Phillips