Trim any silver skin from the beef tenderloin medallions using a sharp knife; marinate the medallions in soy sauce, minced garlic, and brown sugar for 1–2 hours. Let sit at room temperature for 15–20 minutes.

Place a large skillet over medium-high heat and add 2 tablespoons of unsalted butter. Allow it to melt and become bubbly, ensuring it's hot enough to quickly brown the medallions.

Carefully sear the beef tenderloin medallions for 3-4 minutes on one side until a golden-brown crust forms. Flip and cook for another 3-4 minutes to reach desired doneness; use a meat thermometer for accuracy.

Lower the heat and add another tablespoon of butter to the skillet, followed by 2 minced garlic cloves. Cook until fragrant, then stir in finely chopped fresh herbs.

Return the medallions to the skillet, coat with sauce for about a minute, then remove from heat and let rest for 5–10 minutes before serving.