Preheat oven to 400 degrees F.
Whisk together lemon juice and mustard. Add oil in a slow stream, whisking until emulsified. Stir in tomatoes, olives, capers, and salt and pepper, to taste.
Arrange the fish steaks in a ceramic or glass baking dish just large enough to hold them in 1 layer. Pour the vinaigrette over fish.
Cook the fish in middle of oven until just cooked through and no longer translucent, 7 to 9 minutes.
Serve fish with the vinaigrette from the baking dish.