Preheat the oven to 375 degrees F.  Bring the lamb to room temperature, about 30 minutes out of the fridge, and pat it dry with paper towels. In a small bowl mix together the dried oregano, lemon zest, garlic and salt and pepper.

In a large skillet (large enough to hold the lamb roast), heat the olive oil over medium high heat. Sear the roast on all sides, turning so that it browns all over the exterior, about 12 minutes in total. Transfer the pan to the oven and roast until the meat registers 125 degrees F in the center of a roast (for medium rare), using an internal thermometer, about 20 to 24 minutes (start checking at 20).

While the meat cooks, make the Spinach and Herb Salad. In a medium bowl combine the spinach, dill, parsley, oregano. Drizzle over the olive oil and lemon juice, and season with salt and pepper. Toss well.

Make the Garlicky Yogurt. In a small bowl, combine the Greek yogurt, lemon juice, minced garlic, coriander and salt and pepper.

When the lamb is cooked, transfer the roast to a cutting board, sprinkle it with the remaining tablespoon of lemon juice and let it rest for 20 minutes. Cut into very thin slices and place on a very large serving platter or board. Place the roasted tomatoes, sliced onions, radishes (if using), yogurt sauce and spinach salad into serving bowls. Place the pita in a separate bowl or basket. Let everyone assemble their own gyros.