Suppers: [thursday] Greek Beef Pita Pockets

• 4 pita breads
• 4 lettuce leaves
• 1 egg
• 1 onion
• 1 garlic clove
• 450 gr beef (lean ground)
• 160 ml extra yogurt (thick plain)
• 30 ml wine vinegar (red)
• 16 ml vegetable oil
• 7 ml lemon juice