Preheat oven to 450°F (232°C)
Pat the london broil dry and season both sides with salt and black pepper
Add optional marinade or rub:
Garlic, olive oil, soy sauce, Worcestershire, and black pepper
Or olive oil, minced garlic, rosemary or thyme, salt, and pepper
Marinate 4 to 24 hours (optional) or apply dry rub and rest 30 minutes
Heat a cast-iron skillet or grill pan over high heat until very hot
Sear the steak 2 to 3 minutes per side to form a crust
Transfer the skillet to the oven
Roast to desired doneness (use a meat thermometer):
125°F (52°C) for medium-rare (about 6 to 10 minutes depending on thickness)
135°F (57°C) for medium (about 8 to 12 minutes)
145°F (63°C) for medium-well (about 10 to 14 minutes)
Remove from oven and rest 10 to 15 minutes
Slice thinly against the grain
Serve immediately