Arrange lettuce leaves on a large plate. Add all the remaining salad ingredients in small groups (for people to help themselves). Serve peanut sauce separately.
To make peanut sauce, heat oil in a wok or frying pan over high heat. Stir–fry peanuts until light golden brown, about 4 minutes. Remove with a slotted spoon and place on paper towel to cool. Pound or process peanuts until finely ground. Discard oil from pan, reserving 1 tablespoon.
Crush garlic and spring onions in a mortar and pestle with a little salt. Fry in reserved oil, about 1 minute. Add chilli powder, sugar, soy sauce and water. Bring to a boil; add ground peanuts. Simmer, stirring occasionally, until sauce is thick, about 10 minutes. Add lemon juice and more salt, if needed. Cool. (Sauce can be made ahead and stored in a jar in the refrigerator for up to 1 week.)
a bite of history
Gado gado ingredients vary from region to region. Other additions are spinach, snow peas or cauliflower. Firm vegetables can be served raw, blanched or steamed. Whatever method is used, they must remain crunchy. Serve the salad cold or at room temperature.