Combine sugar and fish sauce in a medium saucepan over moderate heat. Stir until sugar dissolves (about 3 minutes) and remove from heat. Whisk in lime juice and let cool.
Heat oil over moderate heat in a small skillet. Add peanuts and cumin and cook until fragrant (about 1 minute).
Spread lettuce in the bottom of a serving bowl. In a separate bowl toss fruit, vegetables, herbs, nuts and seeds with dressing. Pile on top of lettuce and enjoy right away!