Place 1/2 cup romaine and 1/2 cup radicchio on each of 4 salad plates. Divide grapefruit, onion, and avocado evenly among plates; top with pomegranate seeds. If desired, cover and chill salad mixture in an airtight container for up to 1 day ahead of serving.
Combine honey and lime juice, stirring with a wire whisk; cover and chill. To serve, drizzle 1 tablespoon of the dressing over each salad; toss gently to coat.