Tender Slow-Cooked Beef Gyros with Zesty Tzatziki and Crisp Veggies

Ingredients

• 2 tablespoons olive oil
• 1 red onion, sliced
• 1.4–1.8 kg (3–4 lbs) beef chuck roast, cut into 5–7 cm (2–3 inch) chunks
• 4 cloves garlic, minced
• Juice of 2 lemons
• 1 teaspoon dried oregano
• 1 teaspoon salt
• 1/2 teaspoon black pepper
• 8 Greek-style pita breads
• Diced red onion
• Diced tomatoes
• Diced cucumber
• Shredded lettuce
• Tzatziki sauce