Preheat the oven to 425°F
Pat the flap steak dry with paper towels
Season both sides with salt, pepper, and any desired spices
Heat an oven-safe skillet over high heat
Add a little oil to the skillet
Sear the steak for 2 to 3 minutes per side
Transfer the skillet to the oven
Roast for 5 to 10 minutes, depending on thickness and desired doneness
Check the internal temperature with a meat thermometer
Remove at 125°F for rare
Remove at 130°F to 135°F for medium-rare
Remove at 140°F to 145°F for medium
Let the steak rest for 5 to 10 minutes
Slice thinly against the grain
Serve immediately