Step 1

Combine Worcestershire sauce, soy sauce, brown sugar, anchovies, garlic, mustard, tomato paste, and vegetable oil in a blender and blend until smooth, creamy, and emulsified. Transfer 1/3 of marinade to a small container, add chives and shallots, stir to combine, and reserve. Place flank steak inside a gallon-sized zipper-lock bag with remaining marinade. Press out air, seal bag, and allow meat to marinate, turning occasionally, for at least 1 hour and up to 12.

Step 2

Remove steak from marinade and pat dry with paper towels. Ignite a large chimney full of coals and wait until they're covered in grey ash. Spread evenly over 1/2 of grate, leaving the other half empty. Put the cooking grate in place, cover, and allow grill to preheat for 5 minutes. Scrape cooking grates clean, then place flank steak over hot side of grill. cook until well charred, about 3 minutes. Flip steak and continue to cook until second side is well charred, about 3 minutes longer. Transfer steak to cooler side of grill, cover, and cook until the center of the steak registers 125°F on an instant-read thermometer for medium-rare, or 135° for medium. Transfer to a cutting board, tent with foil, and allow steak to rest for at least 5 minutes. Carve and serve, passing extra marinade tableside.

Step 3

Alternatively, to finish indoors: Heat 2 tablespoons vegetable or canola oil in a large stainless steel or cast iron skillet over high heat until shimmering. Remove steak from marinade and pat dryw ith paper towels. Add steak to pan and cook, turning frequently, until an instant-read thermometer registers 125°F for medium-rare, or 135° for medium, reducing heat as necessary if steak smokes excessively or starts to burn. Transfer to a wire rack set in a rimmed baking sheet and allow to rest 5 to 10 minutes. Carve and serve, passing extra marinade tableside.