Preheat the oven to 400 degrees.
Heat oil in a large skillet over medium heat.
Add the onion and cook for 5 minutes until softened.
Stir in the garlic and cook for a minute more, then add the beef to the pan.
Cook the beef while breaking up with a spoon until browned, about 10 minutes.
Turn off the heat and let cool slightly.
Pour in the tzatziki sauce and stir to combine.
Place the pita bread on a sheet pan and toast in the oven until slightly browned and crisp, about 5 minutes.
Remove from the oven and let cool on the sheet pan.
Add the beef topping to the pita chips then cover with the mozzarella cheese.
Bake for 15 minutes until the cheese is melted and remove from the oven.
Top with the tomatoes, cucumber and olives.
Serve while warm.