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 the 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.