For the Mắm Nêm-Pineapple Dressing: To a Vitamix blender, add all ingredients; purée.
To Assemble and Serve: Across a large serving plate, arrange sliced beef in a single layer, covering the entire plate.
Spoon Mắm Nêm-Pineapple Dressing over beef, spreading the dressing evenly.
In a mixing bowl, toss to combine watercress, rau om, onions, peanuts, shallots, and vinaigrette.
Spread salad over beef, leaving the edges of the beef exposed.
Serve with chips or crackers.
