Season steaks with salt and pepper, and place in a baggie with garlic, rosemary, Worcestershire and EVOO; marinate several hours. Bring to room temp, grill or griddle over medium-high heat for 10 minutes turning once, let stand a few minutes and thinly slice on an angle against grain.
For salad dressing, combine sour cream, lemon juice, hot sauce, horseradish, chives, parsley, salt and pepper.
Cover potatoes with cold water and bring to a boil; salt water and cook potatoes to tender, 7-8 minutes. Drain and return to pot to dry, cool potatoes to room temp and toss in dressing along with radishes, celery and onions.
Serve sliced steak on or alongside potato salad.