Generously season steak with salt and black pepper. In a small bowl stir together creme fraiche, horseradish, and vinegar; season lightly with salt and black pepper.
In an extra-large heavy skillet heat 1 tablespoon of the olive oil over medium-high heat; add steak. Cook 12 to 14 minutes or until desired doneness (145 degrees F for medium), turning once. Let rest, covered with foil, on cutting board 10 minutes.
Wipe skillet clean, then heat 2 teaspoons of the olive oil over medium heat. Working in batches so you don't crowd pan, cook potatoes in a single layer until browned and just tender, about 2 minutes a side. Repeat with two more batches using an additional 2 teaspoons olive oil each time. Season with salt and black pepper.
Spread toast with creme fraiche mixture. Slice steak across the grain; top toast with potatoes, steak, and parsley.