Whisk together 4 garlic cloves, thinly sliced, ¼ cup extra-virgin olive oil, 2 Tbsp. good-quality balsamic vinegar, 2 Tbsp. soy sauce, and 1 Tbsp. Worcestershire sauce in a medium bowl or baking dish; season with freshly ground black pepper. Whisk in ¾ tsp. baking soda (don’t worry if marinade fizzes; that’s the baking soda working).

Prick one 1½–2 lb. London broil all over with a fork and add to marinade; turn to coat. Let sit at room temperature at least 1 hour or cover and chill up to 12 hours.

Heat 1 Tbsp. vegetable oil in a large cast-iron skillet over medium. Remove steak from marinade, letting excess drip back into bowl, and pat dry with paper towels; reserve marinade. Season steak generously with kosher salt and cook, turning once, until deeply browned and an instant-read thermometer inserted into thickest part registers 125° for medium-rare, 8–12 minutes. Using tongs, stand steak on its sides and cook until lightly browned, about 30 seconds per side. Transfer to a cutting board and let rest 10 minutes.

Meanwhile, transfer reserved marinade to a small saucepan, add ⅓ cup water, and bring to a boil (mixture will foam up). Cook over medium-high heat, stirring constantly to avoid overflow, until foaming subsides, about 3 minutes. Add 2 Tbsp. unsalted butter and whisk vigorously to emulsify sauce. Keep warm until ready to serve.

Thinly slice steak against the grain and arrange on a platter. Spoon sauce over; sprinkle with flaky sea salt.