Place the tomatoes, garlic, 1 tablespoon of the oil, vinegar, thyme, 1/4 teaspoon salt, and a few grinds of black pepper in a blender or food processor fitted with the blade attachment. Pulse a few times until blended and saucy but still a little chunky; set aside.
Use a paper towel or clean dish towel to pat each salmon fillet dry on both sides (to prevent them from sticking to the pan). Season on both sides with salt and pepper.
Heat the remaining 2 tablespoons of oil in a large skillet with a lid over medium-high heat until shimmering. Place the salmon skin-side down in the skillet, then press down on them so that the skin is in even contact with the pan and browns evenly. Reduce the heat to medium-low and cook without moving, gently pressing down on fish every so often, until the sides are cooked halfway up the fillets, 6 to 9 minutes, depending on the thickness of your fillets. Transfer the salmon skin-side up to a plate.
Pour the tomato mixture into the skillet and bring it to a simmer. Return the salmon skin-side up to the pan, nestling them in the sauce. Cover and continue to simmer for 2 to 5 minutes, depending on the thickness of your fillets. An instant-read thermometer into the middle of the thickest fillet should read 120°F to 130°F for medium-rare or 135°F to 145°F if you prefer it more well-done. Serve the salmon, skin-side up, with plenty of its sauce, over cooked couscous.