Preheat oven to 425 degrees. Lightly grease or spray an 8- by 8-inch baking dish with cooking spray.
In a medium saucepan, combine red wine vinegar, salt and 1 cup water and bring to a boil. Stir in red onion and remove from heat. Set aside to cool.
In the prepared baking dish, toss potatoes with olive oil and kosher salt to taste. Bake, stirring occasionally, for 35 to 40 minutes or just until potatoes are tender when pierced with a fork (don’t overcook). Remove from oven and set aside.
While the potatoes bake, prepare the dressing: In a medium mixing bowl, whisk together the sour cream, yogurt, balsamic vinegar, horseradish, chives and paprika. Season to taste with salt and pepper. Cover and refrigerate until ready to serve.
Preheat grill. Season steak on both sides with salt and pepper. Place on greased grill rack and grill to desired doneness (130-135 for medium rare or 135-145 for medium). Remove from grill and let rest for at least 5 minutes; thinly slice crosswise into 1 to 2-inch pieces. Set aside.
Lightly toss the lettuce with some of the dressing and arrange on a large serving platter. On top of the lettuce arrange the potatoes, steak, chopped eggs, and tomatoes in rows. Drain red onions, sprinkle over the top. Season to taste with salt, pepper and paprika. Serve, passing remaining dressing on the side.