Heat olive oil in a large skillet over medium-high heat.
Add the ground beef to the skillet and cook for 5-7 minutes, breaking the meat into small crumbles with a spatula until fully browned.
Stir in the diced yellow onion and garlic, and sauté for 3 minutes until the onion is translucent.
Add cumin, coriander, paprika, salt, and black pepper; stir thoroughly and cook for 2 minutes to allow the spices to bloom.
Remove the skillet from heat and drain any excess liquid or fat.
Warm the pita halves in a toaster or a 350°F oven for 2-3 minutes until soft and pliable.
Open each pita pocket and distribute the beef mixture evenly among the 8 halves.
Stuff each pocket with shredded lettuce, diced tomatoes, cucumbers, and red onion.
Top each pita pocket with a tablespoon of tzatziki sauce and serve immediately.