• Step 1: In a large resealable bag or shallow dish, combine the sliced beef, olive oil, garlic, oregano, cumin, paprika, lemon zest, salt, and pepper. Toss to coat the beef evenly. Cover and marinate in the refrigerator for at least 30 minutes, or up to 4 hours.
• Step 2: In a medium bowl, mix together the Greek yogurt, diced cucumber, lemon juice, minced garlic, and chopped dill. Season with salt and pepper to taste. Cover and refrigerate the tzatziki sauce until ready to serve.
• Step 3: Preheat a grill or grill pan to medium-high heat. Cook the marinated beef in batches, flipping occasionally, until browned and cooked through, about 5-7 minutes per batch.
• Step 4: Warm the pita or flatbread according to package instructions. To assemble the gyros, place the grilled beef on the warm bread and top with the tzatziki sauce, diced tomatoes, sliced onions, and any other desired toppings.