Rosemary Rack of Lamb with Easy Tzatziki

Name: Rosemary Rack of Lamb with Easy Tzatziki
Ingredients: 1 1/2 tablespoons chopped fresh rosemary leaves; 1 1/2 tablespoons chopped garlic (4 cloves); 1 tablespoon dried oregano; Kosher salt and freshly ground black pepper; 2 tablespoons good olive oil; 2 racks of lamb (14 to 16 ounces each), frenched; Easy Tzatziki (recipe follows); 1 hothouse cucumber, unpeeled and seeded; 2 (7-ounce) containers Greek yogurt, such as Fage Total; 1/4 cup sour cream; 2 tablespoons freshly squeezed lemon juice; 1 tablespoon white wine vinegar; 1 tablespoon minced fresh dill; 1 1/2 teaspoons minced garlic (2 cloves)