Feta isn't the easiest cheese to slice, but just do your best. Crumbling it works just as well.
The reason I use arugula on this sandwich is because the peppery taste is very complimentary to the feta-tomato combination.
This sandwich is perfect for ladies luncheons, book clubs, or anytime you want to make a very simple but very elegant presentation.
* NOTE: This is when a nice bottle of olive oil is called for. You can use your plain old big bottle for other cooking, but when you drizzle olive oil on a sandwich, make sure it's a "top shelf" type. Spend the little extra money - you'll notice a difference.