Cut flounder fillets into 1 1/2-inch strips; pat dry. Place in a medium bowl. Add cornmeal; season with salt and pepper. Toss to coat.
Heat oil in a large nonstick skillet over high heat. Brown fish (work in batches if needed) on both sides, 7 to 10 minutes (turn fish carefully). Transfer to a paper towel-lined plate; season with more salt, as desired.
Split baguette; hollow out slightly. Spread both halves with tartar sauce, and layer with lettuce, sliced tomato, and fish. Cut into 4 pieces.