Peeling the cucumber is optional, I like to do a "rustic peel", meaning a sloppy half peeled job. This way you get some green bits throughout the sauce and it just looks pretty.
Slice the peeled cucumber in half the long way so that you have 2 boats. Run a spoon through the center to remove the juicy seeded center leaving you with boat looking cucumbers. Grate the cucumber using a cheese grater. Put the pile of grated cucumber in a couple paper towels, and squeeze the liquid out over the sink.
In a medium bowl, combine the grated cucumber and the rest of the sauce ingredients. Cover and refrigerate. The longer it sits, the better the flavors come together. I like to make at least an hour before serving.
If using frozen meatballs, add meatballs to a microwave safe bowl, add a little water for steaming, cover with paper towel and microwave until heated through, a couple minutes.
Spread some of the Tzatziki sauce on a warmed pita, then lettuce, tomato, and onion. Arrange 3-4 meatballs on the pita, dollop with more sauce and sprinkle with feta cheese crumbles and some hot sauce!
I suggest making the Tzatziki at least an hour before serving so the flavors can come together, it can even be made the night before.
Have extra sauce? Use it as a fresh veggie dip, yum!
The Joseph's low-carb pitas are so good, and they come in a regular size, and minis! I find them at a kiosk near the deli in my local store.
This is great with grilled or rotisserie chicken as well, I often do a chicken version for round two leftovers for lunch the next day.