If you didn't already ask your fish monger to portion the fish (they're usually happy to do so), cut it into 6 (approximately 5 1/4-ounce) portions, leaving the skin on. Place them in a baking pan.
Add the pinch of saffron threads to a mortar.
Use the pestle to grind the spice until it's a powder. This should only take a few seconds.
Add the orange juice to the mortar and then drizzle the mixture evenly over the fish fillets.
Cover with plastic wrap and place in the refrigerator for 30 minutes to marinate.
Coat the bottom of a large sauté pan with olive oil and place it over high heat. Sprinkle the marinated fish with salt and pepper. Once the pan is very hot, add the fish fillets (do not crowd the pan, so only add as many as will fit with at least 2 -inches between them -- you can always do a second batch). You should hear sizzling when the fish hits the pan -- if you don't, it's not hot enough yet. Sauté just until the fish is cooked through, about 5 minutes. After about the first minute, cover the pan with foil or a lid. (If you don't like the crispy skin, it will easily slide off once the fish is cooked.)
For the salsa
Combine the onion, tomato, basil, sherry vinegar and oil in a small sauté pan. Place the pan over medium heat and cook only long enough to warm it, about 2 minutes. Season to taste with salt and pepper. (Here's How to Season to Taste.) Try to time the fish and salsa so they're ready at about the same time!
Add the fish to a large platter, or individual serving plates and spoon some of the salsa over each portion.