In a large nonstick skillet, heat oil over medium-high heat. Crumble in beef and add ginger; cook 7 minutes, stirring occasionally. Stir in 2 tbsp of the soy sauce and allow to cool.
In the bottom of a 14-cup trifle dish, place lettuce and pack down slightly. Top with a layer of red pepper followed by layers of cilantro (packing down each), cooled beef mixture, edamame, cucumber and scallions.
In a small bowl, whisk peanut butter with 2 tbsp warm water and remaining 2 tbsp soy sauce until smooth. Stir in garlic and mayonnaise.
Spread peanut butter dressing over top of salad. Garnish with chopped peanuts and, if using, cilantro. If desired, toss salad before serving.