Brush the bottom and sides of a 9-by-13-inch rimmed baking sheet with 2 tablespoons of the olive oil. Stretch the pizza dough to fit the pan. Using a pastry brush, brush the dough with the remaining 1 tablespoon olive oil. Evenly distribute the clementine slices, shaved fennel and fennel seeds over the dough, pushing them gently into the dough. Sprinkle with the flaky sea salt and drizzle with an additional teaspoon of olive oil if desired.
Bake the focaccia until golden brown, 25 to 30 minutes. Cut into slices and serve.