How to Make It
In a small skillet, melt the butter. Add the coriander, all the peppercorns and the crushed red pepper and cook over moderately low heat for 1 minute. Scrape into a medium bowl and stir in the brown sugar and 1 1/2 tablespoons of salt. Rub the mixture all over the steak and transfer to a baking dish. Cover with plastic wrap and refrigerate for 3 hours.
In a large saucepan of salted boiling water, blanch the leeks until just tender, 3 to 4 minutes. Using tongs, transfer them cut side down to a paper towel–lined baking sheet to drain.
In a small bowl, whisk the mustard with the vinegar.
In a large cast-iron skillet, heat 1 tablespoon of the oil over high heat until smoking. Add half of the steak and cook over high heat, turning a few times, until lightly charred on the outside and medium-rare within, 4 to 5 minutes total. Transfer to a carving board, spread with the Dijon vinegar and let rest for 10 minutes. Repeat with 1 tablespoon of the oil and the remaining steak and Dijon vinegar.
Wipe out the skillet and heat the remaining 1 tablespoon of oil in it. Add the leeks cut side down and cook over high heat until lightly charred on the bottom, about 3 minutes. Transfer to a platter and drizzle with the lemon juice. Slice the steak against the grain and serve with the leeks.