In a small bowl, combine fish sauce, sugar, ginger and chili-garlic paste with about 1/4 cup water to produce a thin sauce. Taste and adjust seasoning, adding salt and black pepper if necessary.
Divide the lettuce among 4 plates. Top each with a quarter of the noodles, carrots, herbs and shrimp or pork. Drizzle dressing over all, and, if you like, top with Scallion Oil. Makes 4 servings.
Heat oil in a small pan until it is thin and fragrant; stir in salt and sugar. Put scallions in a small bowl and pour the oil over them; they should sizzle. Cool and use by the spoonful. Makes about 1/4 cup.