In a mixing bowl, combine Greek yogurt, finely chopped cucumber, minced garlic, lemon juice, and a pinch of salt. Stir until blended. Cover and refrigerate for 15 minutes.

Heat a large skillet over medium-high heat, add ground beef, and cook for 7-10 minutes until browned. Drain fat, then add chopped mushrooms, salt, pepper, and seasonings. Sauté for 5 minutes until mushrooms are tender.

Sprinkle cheese over the beef and mushroom mixture. Reduce heat to low and stir until cheese melts for 2-3 minutes.

Wrap pita bread in foil and warm in a preheated oven at 350°F (175°C) for 5-7 minutes until soft.

Cut warm pita in half to create pockets. Fill each half with beef and mushroom mixture, then drizzle with tzatziki sauce.

Serve the stuffed pitas warm with a side salad or sweet potato fries.