Do you know that onions have more sugar than an apple? Naturally sweet, it caramelizes when cooked over a gentle flame.
Slice 2 large onions (1 white and 1 red) thinly and allow to sweat in a non-stick pan drizzled with vegetable oil, over gentle heat, at gas mark 4. It also helps to season with kosher salt in the beginning so that it helps remove the moisture off the onions. You may also add a few sprigs of fresh thyme. Once the onions start to turn golden in color, the caramelization process has begun. I stop while it's golden because it will continue to cook and caramelize in the oven with the pastry. It also has to be said, that this can be a base for onion soup, should you wish to continue caramelizing in the pan (of which, I'll share a recipe one of these days).
Dock your puff pastry with a fork and brush with a beaten egg. Top with another layer of puff pastry that has been equally docked with a fork and brushed with an egg, and continue layering until you have 3 layers. On the fourth layer, assemble only on the edges, mimicking a "frame..."
...like so. Allow to rest in the chiller for 20 minutes.
Then top with the caramelized onions, anchovies, sliced black olives, capers, cherry tomatoes, and fresh rosemary leaves. Season with black pepper. Bake at 200 degrees Celcius for 15-20 minutes.
A good puff pastry rises to the occasion! Remove from the pan and allow to cool on a rack.