Make the dressing: Puree the vinegar, mustard, garlic, anchovies and 2 tablespoons water in a blender until smooth. With the machine running, gradually add the olive oil and puree until smooth. Season the steak with salt and pepper and coat with ¼ cup of the dressing. Cover and refrigerate at least 1 hour or up to 8 hours. Refrigerate the remaining dressing until ready to serve.
Meanwhile, put the potatoes in a medium saucepan and cover with water by 1 inch; season with salt. Bring to a boil, then reduce the heat and simmer until just slightly tender, 8 to 10 minutes. Drain and transfer to a medium bowl.
Preheat a grill to medium high. Drizzle the potatoes with olive oil and season with salt and pepper; toss. Put the shrimp in a separate bowl; drizzle with olive oil, season with salt and pepper and toss to coat. Grill the steak until well marked, 5 to 7 minutes per side for medium rare; transfer to a cutting board. Meanwhile, grill the potatoes until well marked, about 3 minutes per side; transfer to a plate. Grill the shrimp until well marked and just cooked through, 2 to 3 minutes per side; remove to the plate.
Combine the spinach, watercress, frisee and chives on a platter. Drizzle with some of the remaining dressing, season with salt and pepper and toss. Thinly slice the steak. Arrange the shrimp, potatoes and steak in rows over the greens. Drizzle with the remaining dressing.