Notes: You can prepare the French onions up to 1 day ahead or start them just before the steak.
Rinse beef steak and pat dry. Pat salt, pepper, onion flakes, and garlic flakes evenly onto meat.
Pour olive oil into an 8- to 10-inch ovenproof nonstick frying pan over high heat; tilt to coat pan bottom. When hot, add steak and brown on both sides, about 4 minutes total. Supporting meat with tongs, tip steak onto its edge and rotate to brown lightly, about 2 minutes total. As fat in pan accumulates, wipe out with paper towels.
Put pan with steak in a 400° regular or convection oven and bake until meat is done to your taste (cut to test), 8 to 12 minutes for rare or about 25 minutes for well done.
Turn oven heat to broil. Remove pan and wipe out fat with a paper towel. Mound French onions onto meat and sprinkle with cheese. Broil in pan about 6 inches from heat until cheese is bubbling and beginning to brown, 3 to 4 minutes. Remove pan from oven and pour broth around meat, stirring to release browned bits.
Leave meat in pan or use a wide spatula to transfer steak and onions to a warm rimmed platter (pour juices around meat). To serve, cut meat and onions into 4 equal portions; set each on a rimmed plate and spoon juices around meat.
This is phenomenal! I have made it several times over the years and it is always a hit.