Remove steaks from refrigeration 30 minutes prior to cooking to reach room temperature.

Pat the steaks completely dry with paper towels to ensure optimal Maillard reaction.

Season all surfaces of the steaks generously with kosher salt and black pepper.

Heat a heavy cast-iron skillet over high heat until the oil begins to shimmer and reach its smoke point.

Place steaks in the skillet and sear for 3 to 4 minutes without moving to develop a deep brown crust.

Flip the steaks and sear the secondary side for an additional 3 minutes.

Reduce heat to medium-high and add the butter, smashed garlic cloves, and thyme sprigs to the skillet.

Tilt the pan and use a large spoon to continuously baste the steaks with the foaming butter for 2 to 3 minutes until the internal temperature reaches 130°F (54°C) for medium-rare.

Transfer steaks to a cutting board and allow to rest for 10 minutes.

In the same skillet with the remaining fat, add the sliced onions and cook over medium heat, stirring frequently, until softened and golden brown (approximately 8-10 minutes).

Slice the steak against the grain and serve topped with the caramelized onions and remaining pan butter.