Whisk together first 5 ingredients in a small bowl. Combine lettuce, cucumber, and shallots in a large bowl. Stir in 1/4 cup mint and 3 tablespoons cilantro. Stir in all but 2 tablespoons dressing, tossing well.
Toss roast beef with remaining 2 tablespoons dressing. Arrange salad on a platter, and top with roast beef. Sprinkle with peanuts and remaining mint and cilantro.