These splendidly rich tarts are my version of French onion galettes. Buttery, flaky pastry crusts are filled with deeply caramelized onions that are generously laced with sage.
The steps to prepare these tarts may seem familiar, but if you take extra care with them, you'll be amazed by the results. Handle the pastry with precision so that it bakes tender, flaky, and shatteringly crisp; spend the time to slowly and thoroughly caramelize the onions until they melt into a golden marmalade; and give the tarts their final baking as close to serving time as possible.
You'll notice the onions are caramelized in a deep saucepan instead of a wide skillet. It makes them easier to stir without flying out of the pan and gives them a chance to soften and stew in their own liquid before it boils away. Once the liquid evaporates, the onions will concentrate and brown, and the balsamic vinegar works to balance the sweetness of the onions and deepen their color. For the best flavor, the whole process should take at least half an hour. Be sure to use regular yellow onions, not Walla Walla, Vidalia, or other sweeter summer onions—they have too much water and do not caramelize well.
Serve the tarts as an hors d'oeuvre at any elegant occasion, or as an accompaniment to a seasonal salad for a light lunch or supper.
