Considered a savory dish, a perfect onion tart requires that the onions be nicely caramelized. Time is of the essence in making this tart, and cooking the onions cannot be rushed. Though the recipe can a bit involved, if a homemade crust is made by following a few tips, an onion tart can come out of the oven pretty as a picture every time.
The key ingredient when caramelizing onions is time. The cooking process absolutely cannot be rushed. When onions are being caramelized, the sugars within the onions are being turned to caramel. This is why caramelized onions have such a sweet, gentle taste to them and are not as harsh in flavor as quickly cooked onions can tend to be.
To caramelize onions for an onion tart, cut them in slivers of approximately all one size. This detail is important as it will allow for more evenly cooked onions. If the slices are of varying sizes, the onions will not all be caramelized. Instead, some will be burned, while others will not be cooked yet at all.
In a pre-heated frying skillet, add butter or oil. Once the butter or oil is warmed and evenly distributed, add the onions in an even layer. Do not over-fill the pan, or the onions will not cook evenly. Stir constantly so the onions do not burn on one side while failing to cook on the other. The onions will need to cook approximately 15 minutes. Do not let them over-brown, or they will quickly burn. Instead, take them off of the burner when they have reached a deep golden brown.
Never leave the onions in the skillet after they have been removed from the burner. The heat from the still warm skillet will continue to cook them. Transfer them to another dish immediately. If there is any liquid or grease along with the onions, make sure to drain this. If grease and liquids are left with the onions, they will cause the pastry shell to be soggy, defeating the purpose of pre-cooking the shell for the onion tart.
The onions should be placed immediately into the pre-cooked pastry shell. Some recipes call for an egg and cream mixture to be poured over the onions, while others suggest a crumble of gorgonzola to be liberally distributed atop. The onion tart is best served straight from the oven, although some enjoy it presented at room temperature.
