1. Soak shallot in water 10 minutes. Meanwhile, grate 1/2 teaspoon lemon zest, then remove peel and pith from lemon. Working over a bowl, cut between membranes to remove segments. Add drained shallot; season with salt and pepper. In another bowl, stir together butter, peppercorns, and zest; season with salt.

2. Heat a heavy skillet (preferably cast iron) over medium-high; swirl in 1 tablespoon oil. Season steak with salt and pepper; cook, undisturbed, until a dark crust forms, 4 to 5 minutes. Flip; cook about 3 minutes more for medium-rare. Brush with some of butter mixture; transfer to a cutting board and let stand 10 minutes. Slice steak across the grain; season with salt. Toss lemon mixture with frisee, mint, and remaining 1 tablespoon oil. Serve steak with salad, baguette, and remaining butter mixture.