Method:
Put lamb, 3 tablespoons of the oil, oregano, basil, garlic, salt and pepper into a bowl, toss well to coat, cover with plastic wrap and refrigerate for 2 to 3 hours, or overnight.
Preheat grill. Arrange lamb on skewers and grill, turning occasionally, until medium rare. (Alternately, sear lamb in a large skillet over medium-high heat in two batches until medium rare, 5 to 6 minutes.) Transfer to a plate as done and set aside.
Arrange spinach on 4 plates and top with feta, potatoes, tomatoes, cucumbers, capers and olives. Arrange cooked lamb over spinach. In a small bowl, quickly whisk together remaining 6 tablespoons oil, vinegar, mint, salt and pepper to make a dressing, then drizzle over salad and serve.