Steak Frites with L'Entrecôte Sauce

Method

1. Rinse the potatoes thoroughly until the water runs clear.
2. Bring a pot of water to a boil with a big pinch of salt and a splash of vinegar. Add the potatoes and return to a boil, then immediately remove from heat and drain.
3. Once the potatoes are cool enough to handle, cut them into fries (if not already cut).
4. Heat oil for frying to 375°F (190°C). Fry the potatoes until golden and crispy. Drain and season with salt.
5. For the sauce: Melt a few tablespoons of unsalted butter in a pan. Add finely chopped shallots and cook until soft.
6. Add anchovies and cook until they disintegrate. Remove from heat and add more butter, stirring to emulsify.
7. Let the shallot-butter mixture cool completely.
8. In a separate bowl, combine 1 egg yolk, mustard, capers, lemon juice, fresh tarragon, fresh parsley, Worcestershire sauce, and a little sage.
9. Using an immersion blender, blend the ingredients until roughly combined.
10. Add the cooled shallot-butter mixture to the egg yolk mixture and blend again until smooth. Adjust consistency with a splash of water if needed.
11. Season the steak generously with salt on both sides and let it rest at room temperature for about 1 hour.
12. Cook the steak to your desired doneness in a hot pan or grill.
13. Let the steak rest for a few minutes after cooking, then slice.
14. Serve the steak with the fries and drizzle with the sauce.