Preheat oven to 400 degrees. Whisk together lemon juice and mustard. Add in oil in a slow stream, whisking till emulsified. Stir in tomatoes, olives, capers, and salt and pepper, to taste. Arrange the fish steaks in a ceramic or possibly glass baking dish just large sufficient to hold them in 1 layer. Pour the vinaigrette over fish. Cook the fish in middle of oven till just cooked through and no longer translucent/soft, 7 to 9 min. Serve fish with the vinaigrette from the baking dish. This recipe yields 4 servings.