In a medium mixing bowl, combine the unsalted butter (softened), finely minced garlic, chopped parsley, thyme leaves (if using), lemon zest, 1/2 tsp kosher salt and 1/4 tsp black pepper. Mix thoroughly with a fork until homogeneous. Taste and adjust seasoning. Transfer the mixture to a sheet of plastic wrap, shape into a log about 1 inch in diameter, tightly wrap and refrigerate until firm, at least 30 minutes.

Beef and Assembly

Prepare steaks

Pat the steaks dry with paper towels. Using a sharp knife, make a horizontal pocket in the side of each steak about 3/4 the length of the steak and roughly 1/2 to 3/4 inch deep — do not cut all the way through. Alternatively, butterfly the steak leaving a hinge and create a cavity. Lightly brush both sides of the steaks with 2 tbs olive oil and season both surfaces and inside the pocket with 1 tsp salt and 1/2 tsp freshly ground black pepper.

Stuff steaks

Remove the garlic butter log from the refrigerator and slice it into 4 equal-length pieces. Cut each piece into halves if needed to fit the pocket. Insert one portion of the chilled garlic butter into each steak pocket, pressing it gently so it sits inside and doesn't protrude. If the pocket edge opens, fold it over and secure with 1 piece of kitchen twine wrapped around the steak once or twice and tied snugly. Repeat for all steaks.

Sear steaks

Preheat a large ovenproof skillet over medium-high heat until very hot (about 2–3 minutes). Add a small drizzle of oil if pan is dry. Place stuffed steaks seam-side down, sear for 2–3 minutes without moving to develop a deep brown crust. Flip and sear the other side 2 minutes. Spoon any rendered juices and remaining butter that may have escaped into the pan to baste the steaks while searing.

Finish in oven

Preheat oven to 400°F (200°C). After searing both sides, transfer the skillet to the oven (or move steaks to a baking sheet) and roast until internal temperature reaches 125°F–130°F (52°C–54°C) for medium-rare, approximately 6–10 minutes depending on thickness. For medium, cook to 135°F (57°C). Use an instant-read thermometer for accuracy. Remove steaks to a cutting board, tent loosely with foil and rest 8–10 minutes; carryover will raise temperature a few degrees.

Finish seasoning

Remove kitchen twine before serving. Sprinkle a light dusting of smoked paprika (optional) and a small pinch of finishing salt if desired.

Pan Sauce (optional)

Make pan sauce

While steaks rest, place the skillet with the fond over medium heat (if you used a different pan, pour drippings into the skillet). Add the red wine (or 1/4 cup stock) to deglaze, scraping up browned bits with a wooden spoon, and simmer until reduced by about half (2–3 minutes). Add 1 cup beef stock and simmer 3–4 minutes until sauce thickens slightly. Whisk in 1 tsp Dijon mustard. Remove from heat and swirl in 2 tbs cold butter one cube at a time to create a glossy sauce. Taste and season with salt and pepper. Stir in 1 tbs chopped parsley. Spoon sauce over sliced steaks or serve on the side.