Gyro Beef Brisket Sandwich with Lemon-Feta Tzatziki

Heat 2 tablespoons oil in a skillet over medium-high heat. Add brisket; sear 5 minutes per side. Transfer meat to a 4 to 6 quart slow cooker. Deglaze skillet with broth, scraping up browned bits. Add broth and onion to slow cooker. Cover, cook on high-heat setting for 4-5 hours. Remove the meat and onion and set on a plate; using two forks, shred meat. Puree yogurt, feta, cucumber, scallions, zest, lemon juice, oil, oregano, salt, and pepper in a food processor until smooth. To serve, divide the meat and onion among flatbread rounds. Top each sandwich with romaine, tomato, and tzatziki.