Drizzle the olive oil into the bottom of the slow cooker. Place the red onions over the olive oil.
Arrange the roast over the onions. Drop the garlic over the roast and season the pieces with the lemon juice, oregano, salt, and black pepper.
Place the lid over the slow cooker and cook on low for 8 hours.
Remove the lid and shred the beef with two forks.
Warm the pita breads if desired (char them slightly). Serve the beef with a slotted spoon over pitas. Top as desired with onion, tomatoes, cucumber, lettuce, and tzatziki sauce.
Video
Notes
How to char pita bread?Brush both sides of the pita bread with olive oil (spraying with cooking spray works too). Place it in a skillet over medium-low heat and "cook" on each side for 10-15 seconds.How to store:Any leftovers can be stored in an airtight container and kept in the refrigerator for up to five days, or in the freezer for up to three months. Use it to make more gyros later or make something entirely new like beef tacos or shredded beef salad.Can I use a different cut of meat?Yes! Another go-to option for a recipe like this is the flank steak. You can also fully substitute the meat altogether and use chicken to make a chicken gyro or lamb to make lamb gyros.