Combine the olive oil, onions and thyme in a low-sided heavy pan set over medium heat. Cook until the onions are soft , but not browned, about 20 to 30 minutes. Add the salt and cook for a few minutes more, stirring frequently. Pour the onions into a strainer set over a bowl to drain, reserving the liquid. Allow to cool before using.

Preheat oven to 400°F. Roll out the dough into a 14-inch circle. Brush off the excess flour and transfer the dough to a baking sheet lined with parchment paper. Place in the refrigerator to firm up for 10 minutes. (If using puff pastry, cut and then cool in the same way). Remove the thyme stems from the cooled onions and spread the onion mixture over the dough, leaving a 1½-inch border at the edge. Fold the border up over the onions. For a shiny, more finished look, beat the egg with the milk and brush onto the folded dough edges.

Top the onions with as many anchovies as you like, and plenty of black olives. For a traditional French look, arrange the anchovies in a “checkers” pattern, with an olive in the center of each square.

Place the tart on the bottom rack of the preheated oven and bake for 15 to 20 minutes, or until the crust is golden brown on the bottom and the edges are dark brown and crispy. (Note: Puff pastry cooks faster than pizza dough, so check the tart after 15 minutes or so, depending on your oven. It should be very crisp and well browned.) Slide the tart off the pan onto a rack to cool; serve warm or at room temperature.