Bring 2 1/4 cups water to a boil in a medium saucepan; stir in rice. Cover, reduce heat, and simmer 20 minutes or until rice is tender and liquid is absorbed. Combine rice, spinach, and next 3 ingredients in a large bowl.
Combine lemon juice and next 5 ingredients in a small bowl, stirring well with a wire whisk. Pour lemon juice mixture over rice mixture; toss gently. Cover and chill. Stir in yogurt just before serving. Serve on lettuce leaves, if desired.