Heat a gas grill to medium high or prepare a medium-hot charcoal fire with the coals banked to one side to provide a cooler area on the grill.
On a well-floured surface roll out the pizza dough with a rolling pin until it’s 1/8 inch thick. If the dough is very elastic and resists rolling, cover it with plastic and let it rest for about 5 minutes. You may have to repeat this step a few times until the dough is relaxed and willing to roll. Using a 3-inch ring cutter, cut the dough into 18 rounds. Discard the excess dough. Brush the top of the dough rounds with oil and sprinkle with salt, pepper, and fennel seeds, pressing gently to make sure they adhere. Transfer the rounds to a baking sheet, fennel side up.
Working with half of the pizzettas at a time, grill them fennel side down (covered on a gas grill, uncovered on a charcoal grill) for 1 minute. Check the pizzettas: If they have puffed up, flatten them with a metal spatula. Brush the floured sides (which are facing up) with oil. Grill until the bottom is nicely browned and crisp, about 1 more minute. Loosen with a metal spatula, if necessary, and return the pizzettas, grilled side up, to the baking sheet. (If using a gas grill, turn the heat down to medium.)
Working quickly, top each with 2 wedges of Bûcheron, 3 olive pieces, 3 tomato slices, and a pinch of red pepper flakes. Use a small spoon to sprinkle each pizzetta with a few drops of sambuca or Pernod.
Return the pizzettas to the mediumheat gas grill or to the cooler side of a charcoal grill. Continue grilling, covered, until the pizzettas are crisp and the cheese is melted, about 2 minutes. Transfer to a platter and serve while you repeat with the remaining dough rounds.
The pizzetta toppings can be prepared up to 1 day ahead and refrigerated. The dough can be rolled and cut up to 1 hour ahead and refrigerated, covered.