Lamb Burger With Chunky Mint Tzatziki

Ingredients

• 2/3 cup greek yogurt
• 1/2 english cucumber, seeds scooped out, skin peeled off, and cut into chunky half moons
• 1/4 cup roughly chopped mint leaves
• 1 bunch roughly chopped chives (about 1/3 cup), divided
• 3 tablespoons roughly chopped fresh oregano (divided)
• 3 medium cloves garlic, minced (about 1 tablespoon)
• 2 teaspoons fresh juice from 1 lemon
• kosher salt and freshly ground black pepper
• 14 ounces ground lamb
• a drizzle of olive oil
• 2 warm pitas