In a medium bowl, combine the tuna, capers, 3 tablespoons of the oil, the vinegar, and 1/4 teaspoon pepper.
In another bowl, combine the fennel, parsley, 1/4 teaspoon salt, 1/4 teaspoon pepper, and the remaining oil.
Cut the baguette in half lengthwise, then into quarters or sixths crosswise, and divide among individual plates. Serve the tuna and fennel salads over the bread.
Upgrade: Add flavor and richness to this savory sandwich by using Spanish or Italian light-meat tuna (packed in olive oil) instead of domestic tuna.