Combine onion and 1/2 teaspoon oil, tossing to coat. Arrange onion on a grill rack; grill 9 minutes or until browned and tender, turning frequently.
Combine remaining 1 1/2 teaspoons oil, juice, soy sauce, and honey in a small bowl, stirring with a whisk.
Place spinach in a large bowl; sprinkle with pepper and salt. Add onions. Drizzle oil mixture over spinach, tossing gently to coat. Sprinkle with sesame seeds.