1. Combine the first 7 ingredients in a small bowl; stir well.
2. Gently combine beef and lamb, being careful not to overwork. Divide the mixture into 4 equal portions, gently shaping each into a 1/2-inch-thick patty. Press a nickel-sized indentation in the center of each patty. Sprinkle evenly with 1/4 teaspoon salt and pepper.
3. Heat a large cast-iron skillet over medium-high heat. Add oil to pan, and swirl to coat. Add patties; cook for 3 minutes on each side or until desired degree of doneness.
4. Place 1/4 cup spinach, 1 onion slice, and 1 patty on bottom half of each hamburger bun. Top each patty with 2 tablespoons raita and top half of bun.