In a saucepan, sprinkle gelatin over broth; let stand for 5 minutes. Cook over low heat until gelatin is dissolved. Add water, lemon juice and salt; mix well.
Place mayonnaise in a bowl. Gradually add broth mixture, stirring constantly until smooth. Chill until slightly thickened, about 40 minutes. Fold in spinach, eggs, bacon and onions. pour into an greased 6-cup mold. Chill until firm.
When ready to serve, unmold onto a platter; garnish with tomatoes if desired.