1. In a large bowl, combine all marinade ingredients (ground coriander, garlic powder, dried oregano, ground cumin, smoked paprika, kosher salt, lemon juice, and olive oil).
2. Add the lamb cubes and mix until fully coated.
3. Cover and marinate at room temperature for at least 30 minutes, or refrigerate for several hours or overnight for deeper flavor.
4. While the lamb marinades, grate the cucumber using a box grater or food processor.
5. Transfer the grated cucumber to a paper towel or clean kitchen towel and squeeze out excess moisture.
6. In a mixing bowl, combine the drained cucumber with Greek yogurt, fresh dill, mint, grated garlic, olive oil, salt, lemon zest, and lemon juice.
7. Stir the tzatziki until well mixed, then cover and refrigerate for at least 30 minutes (or overnight) to allow the flavors to develop.
8. Preheat your grill to medium-high heat.
9. Thread the marinated lamb cubes and vegetables (such as red onion, zucchini, mushrooms, eggplant, and bell peppers) onto skewers, alternating ingredients for even cooking.
10. Brush the assembled skewers lightly with olive oil to prevent sticking.
11. Increase grill heat to high and place the skewers on the grill. Immediately reduce heat to medium.
12. Grill the kabobs for 8–12 minutes, turning every 2–3 minutes until evenly charred and cooked to your desired doneness.
13. Use an instant-read thermometer to check the internal temperature of the lamb: 145°F (63°C) for medium-rare, 160°F (71°C) for medium, 170°F (77°C) for well-done
14. Once cooked, remove the kabobs from the grill and let them rest for a few minutes.
15. Serve the grilled lamb and vegetables with a generous spoonful of chilled tzatziki. Optional: serve with warm pita, rice, or a fresh salad on the side.