Preheat your oven to 400°F (200°C). If using puff pastry, remove it from the fridge to allow it to soften slightly. Begin by thinly slicing the onions and set them aside.

In a large pan, heat 2 tablespoons of olive oil over medium heat. Add the sliced onions, stirring occasionally. Cook for about 15-20 minutes until they begin to soften and turn golden. Add the garlic, thyme, and sugar, and season with a pinch of salt and pepper. Continue cooking for another 5-10 minutes until the onions are caramelized. Set aside to cool slightly.

If using pizza dough, roll it out into a thin rectangle or circle, about ¼ inch thick, on a lightly floured surface. If using puff pastry, unfold it and place it on a baking sheet lined with parchment paper. Prick the dough or pastry lightly with a fork to prevent puffing during baking.

Spread the caramelized onion mixture evenly over the surface of the dough, leaving a small border around the edges.

Arrange the anchovy fillets in a crisscross pattern on top of the onions. Place a black olive in each intersection of the anchovies. Add capers if desired.

Lightly brush the edges of the pastry or dough with olive oil. This will help achieve a golden crust.

Bake the Pissaladière in the preheated oven for about 20-25 minutes, or until the dough or pastry is crisp and golden, and the onions are deeply caramelized.

Remove from the oven and allow the Pissaladière to cool slightly before slicing. Serve warm or at room temperature. This dish pairs beautifully with a crisp green salad or as part of an appetizer spread.