Divide the lettuce among individual plates and drizzle with 2 tablespoons of the oil.
In a bowl, combine the tuna, beets, scallions, parsley, salt, pepper, and remaining oil. Spoon alongside the lettuce.
Slice the baguette in half lengthwise, then cut each half crosswise into 4-inch-thick pieces. Spread the cut side of each piece with the tapenade. Serve with the lettuce and tuna.