Remove the sirloin steaks from the refrigerator 20 to 30 minutes before cooking. Pat the steaks completely dry with paper towels.
Season both sides of the steaks generously with kosher salt and freshly ground black pepper.
Heat a cast-iron skillet or heavy-bottomed skillet over medium-high heat for 3 to 5 minutes, until very hot.
Add the oil and heat until it shimmers.
Carefully place the steaks in the skillet. Do not move them for 3 to 4 minutes, until a deep golden-brown crust forms.
Flip the steaks and cook the second side for 2 to 4 minutes, depending on thickness and desired doneness.
Reduce the heat to medium. Add the butter, smashed garlic, rosemary, and thyme to the skillet.
When the butter melts and begins to foam, tilt the skillet slightly. Use a spoon to baste the steaks continuously with garlic-herb butter for 1 to 2 minutes.
Check the internal temperature using an instant-read thermometer. For USDA minimum food safety guidance, cook whole cuts of beef to 145°F / 63°C and rest for at least 3 minutes.
Transfer the steaks to a cutting board or warm plate. Rest loosely covered with foil for 5 to 10 minutes.
Slice the steak against the grain. Spoon the garlic-herb butter over the sliced steak and finish with flaky sea salt, if desired.