Preheat the broiler. Make shallow cuts in the chicken thighs with a sharp knife. Toss the chicken with the lemon juice and 1 1/2 teaspoons salt in a large bowl.
Pulse 2 tablespoons yogurt, the vegetable oil, onion, garlic, ginger, tomato paste, coriander, cumin, 11/2 teaspoons paprika and 1/2 teaspoon salt in a food processor to make a paste. Toss the chicken in the mixture and let marinate 15 minutes.
Place the chicken on a foil-lined broiler pan. Broil, turning once, until slightly charred and a thermometer inserted into the center registers 165 degrees F, 5 to 6 minutes per side.
Meanwhile, combine the remaining 1/2 cup yogurt and 1/4 teaspoon paprika, the cilantro and a pinch of salt in a bowl. Top the chicken with the yogurt sauce and serve with rice, if desired.
Per serving: Calories 237; Fat 9 g (Saturated 2 g); Cholesterol 115 mg; Sodium 1,266 mg; Carbohydrate 8 g; Fiber 2 g; Protein 30 g