In a heavy saucepan over medium heat, combine tomato juice with ground red pepper, salt, black pepper and sugar (if using). Add agar flakes. Cook and stir mixture until agar is completely dissolved. Remove from heat and let cool 15 minutes. Stir in chopped onions, celery or cucumber and minced chili peppers. Pour mixture into lightly greased 5-cup mold and refrigerate until chilled and set. Serve slices on a bed of shredded romaine lettuce, with a generous dollop of Avocado-Lime Mayonnaise.

For Avocado-Lime Mayonnaise: In a food processor, blend avocados with garlic until smooth. Add lime juice and process until thoroughly combined. With machine running, gradually add oil to mixture until well blended and thick. Season to taste with salt and sugar. Refrigerate until ready to serve. Just before serving, blend in chopped cilantro.