Preheat the oven to 450 degrees Fahrenheit. Sprinkle fish evenly with salt and pepper.

Heat oil and margarine in a 12-inch ovenproof skillet over medium-high heat. When margarine has melted, add shallots and garlic and cook for about four minutes, or until shallots are softened. Add lemon slices and cook for about one minute. Flip lemon slices. Add thyme sprigs and wine to the skillet and bring to a boil. Turn down to simmer and gently place salmon in cooking liquid. Poach for about six minutes. Add tomatoes and place the skillet on the top rack of the oven. Roast until the fish is lightly browned on top and flaky, about four minutes.

Before serving, remove fish from liquid. Heat the cooking liquid with the shallots, lemon, and garlic over medium-high heat and cook for a few minutes until slightly reduced. Add kosher salt and pepper to taste (about 1/2 teaspoon kosher salt and 1/4 teaspoon black pepper).

Serve fish warm or at room temperature, with shallots, lemon, and garlic and sauce.