Marinate salmon, if desired, while grill is heating. Grill salmon, skin side up first, 2 or 3 minutes per side. While grilling . . . Prepare sauce. This is a quick process taking care not to overcook tomatoes. They should retain some of their shape. In frying pan, saute garlic and onion just until tender. Add tomatoes, parsley and cilantro and mustard. Cook just until tomatoes are heated through and remove from heat. Remove salmon from grill, top with sauce and serve. Serve remaining sauce in a bowl. The remaining sauce makes a wonderful dip for crusty bread served alongside the fish. A southwestern variation -- add a handful of frozen corn along with the tomatoes and a half-teaspoon or so of chile powder. Serves 4.