In the same skillet, add ground beef and cook 6–8 minutes, breaking it up until browned and cooked through. Stir in garlic powder, dried oregano, salt, and pepper.
Return mushrooms to the skillet with the beef, mix well, and remove from heat.
Preheat oven broiler (or toaster oven). Split pita breads and open pockets carefully.
Fill each pita pocket equally with beef–mushroom mixture, then top with shredded cheese.
Broil for 2–3 minutes, or until cheese melts and edges are lightly golden. Watch closely to avoid burning.
Remove from oven, drizzle or spoon tzatziki sauce into each pocket, garnish with parsley if using, and serve immediately.
Notes
You can substitute ground lamb or turkey for the beef.
Mix in chopped onions or bell peppers with mushrooms for extra flavor.
Make tzatziki from scratch or use store‑bought for convenience.
To add heat, include a pinch of red pepper flakes in the beef mixture.