Directions

1. Steam the potatoes until tender, 12-15 minutes. Toss with 2 tbsp of the butter and season with 1/2 tsp each salt and pepper.
2. Meanwhile, heat the oil in a large skillet over medium-high heat. Season the steak with 1/2 tsp each salt and pepper and cook until medium-rare, 3-5 minutes per side. Transfer to a cutting board and let rest for 5 minutes, then slice across the grain.
3. Return the pan to medium heat. Add the shallots, cornichons, mustard seeds, and the remaining butter. Cook, stirring often, until reduced by about half, 2-3 minutes. Stir in the vinegar, parsley and 1/4 tsp each salt and pepper. Serve the sauce over the steak and potatoes.