Combine the first 7 ingredients in a bowl; cover and store in fridge until you cook the fish. Place the buns in a 200 degree F oven to warm. Heat the oil in a large, non-stick skillet set over medium-high heat. Season the halibut with salt and pepper and pan-fry 3 to 4 minutes per side, or until just cooked through. Slice the buns open and spread with the tartar sauce. Place a lettuce leaf on the bottom half of each bun, top with the fish, tomato, top half of bun and serve.