Some of the best tips for baking a key lime tart are to use fresh key limes and to make the crust from scratch. Bottled Key lime juice can be used as well, but the tart flavor is usually better using fresh limes. This open-top pie can be made in a large tart pan or in smaller, individual sizes. It also is important not to over-bake the tart, and letting it sit and cool before refrigerating it can help it set better.
To make a key lime tart, the first step is to make the crust. Some recipes call for puff pastry, while others recommend a graham cracker crust. Whichever pastry shell is chosen, it typically is best to make it from scratch and to pre-bake it before filling. While there are many ready-made puff pastries and crusts available on the market, making a crust from scratch is not as time consuming as it seems, and the outcome usually is worthwhile.
The next step is to make the key lime filling. While there are decent key lime juices available in some supermarkets and specialty stores, nothing beats the real thing. Try to find Key limes in season and look for light yellow skin that is free of spots and discoloration. Key lime tart recipes will call for anywhere between four and 20 Key limes to get the required amount of juice.
Key lime tart recipes vary greatly in terms of how to make the filling as well. Some call for sweetened condensed milk and eggs along with the lime juice to make a quick, thick filling. Others go the route of filling the tart with Key lime curd, which is easily made by whisking together eggs, yolks, sugar and Key lime juice over a double boiler. For added flavor, finely grated lime peel can also be mixed in, and for a stronger green color, a few drops of food coloring can even be added.
After filling the pastry shell with the Key lime filling, it should be baked for about 20 minutes — it's important not to overcook the tart. Another tip is to remove the tart from the oven, let it cool to room temperature, and chill it in the refrigerator for an hour or overnight. This will help the filling set. Once cooled the key lime tart can also be topped with meringue, but this is optional.