Grate the shallot and finely dice the capers and cornichons. Mix them with the Dijon mustard, hot sauce (e.g., Tabasco), Worcestershire sauce, and parsley. Season with flaky salt and cracked black pepper.
Cut the fillet steak into small, evenly sized pieces using a sharp knife. Combine it with the tartare seasoning mixture and refrigerate while prepping the egg yolk.
Heat a neutral oil in a saucepan, ensuring the depth is around 3 inches.
Combine the panko breadcrumb ingredients and carefully place an egg yolk into it. Toss the panko over the yolk with a fork and then lift it out and into the hot oil.
Deep fry each egg yolk for 10-15 seconds or until the panko is golden, then gently remove to a paper towel-lined plate with a fork.
Serve the steak tartare on a piece of toast, topped with the panko fried egg yolk.
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Notes
To make this into a main course, double the quantities, so allow for 150g of steak per person (5 ounces) Do not make ahead and store - this dish needs to be eaten as soon as possible after assembly. If you have leftovers, fry them up into patties and cook through.