Preparing the sauce.

In a medium pot (preferably a Dutch oven), heat the olive oil over medium. Add the onion and cook, stirring regularly, until just beginning to brown, about 5 minutes. Add the garlic and cook 1 minute more, stirring several times. Raise the heat to medium-high and add the tomatoes, thyme, lemon zest and half of the olives, capers and chiles. Simmer briskly, stirring frequently, for about 5 minutes to evaporate some of the liquid. Reduce the heat to medium-low, stir in 1 cup of water and simmer for 15 minutes. Taste and season with salt, usually about 1 teaspoon. Remove from the heat to cool

Grilling the fish.

Heat a gas grill to medium-high or light a charcoal fire and let it burn just until the coals are covered with gray ash and very hot. Reduce the heat on one side of the gas grill to medium-low or set up the charcoal grill for indirect cooking by banking all the coals to one side, leaving the other half of the grill empty. Set the cooking grate in place, cover the grill and let the grate heat up, 5 minutes or so.
If desired, use a wooden pick to skewer the ends of each salmon steak together so they don't tear off during the grilling. Brush or spray both sides of the salmon steaks with oil and sprinkle with salt. Lay the salmon steaks over the hottest part of the fire and cook for about 4 minutes, until nicely browned underneath. Using a spatula, carefully flip the fish over onto the cooler side of the grill for 2 to 4 minutes to get the salmon to that medium-rare, still-slightly-translucent-in-the-center stage that I love. (Or judge your own preference for doneness accordingly.)

Cooling the fish in the sauce.

Spoon the sauce into a deep platter and nestle the fish in it. Let stand at room temperature for about an hour to bring together the flavors of fish and sauce

Serving.

Sprinkle the fish with the remaining olives, capers and chiles and decorate with thyme sprigs, and you're ready to serve

working ahead: Everything about this dish says "make ahead." The sauce will keep for several days in the refrigerator, well covered. In fact, the whole dish can be made a day ahead without much compromise in quality. simply wrap it and refrigerate. Let the fish and sauce warm to room temperature before serving. If you make the dish early on the day you're serving, don't let it stand at room temperature any longer than 2 hours without refrigeration.