There are many ways to enjoy cheese, but this tart gives you the flexibility to choose your favorites. Which ones to use is entirely up to you, but a mixture of three different varieties will offer up a nice mix of flavor and texture. A soft goat, brie, or blue always adds some punch and saltiness, while an aged cheddar brings gooey, meltiness. The tart is also a great way to use up leftover cheeses from your cheeseboard. Not only can you change the cheeses to suit your taste, but it's easy to customize with your favorite herbs and even some spices for an aromatic kick. The base of this tart is the much-loved caramelized red onion mixture. It's always a winner, so you may want to make extra filling as it freezes really well.