- Combine yogurt, olive oil, lemon juice, garlic, and white pepper in a bowl for the tzatziki sauce. 
- Cover and refrigerate the yogurt mixture for one hour. 
- Blend the chilled yogurt mixture with a whip until smooth. 
- Dry the diced cucumbers by patting with paper towels. 
- Add cucumbers and dill to the yogurt mixture, then season with salt and pepper to taste. 
- Mix lamb and beef together in a glass bowl. 
- Crumb the day-old bread into the meat mixture. 
- Add all spices (garlic, parsley, salt, pepper, fenugreek, and cumin) and mix well. 
- Stir the beaten egg into the meat mixture until well combined. 
- Form the mixture into 2 large patties. 
- Place patties on a medium-hot grill and cook for 5 minutes per side or until cooked through. 
- Slice the cooked gyros meat and serve on warm pita bread with fresh sliced tomatoes, onions, and tzatziki sauce.