Season the pork medallions with salt and pepper on both sides.
Heat olive oil in a large skillet over medium-high heat. Add the pork medallions and cook for approximately 3 to 4 minutes on each side or until they are browned and cooked thoroughly. Once done, transfer them to a plate and cover them with foil to keep them warm.
In the same skillet, melt the butter over medium heat. Add the thinly sliced onion and cook until they are soft and caramelized about 8 to 10 minutes.
Add the minced garlic to the skillet and cook until fragrant, about 20 seconds. Sprinkle the all-purpose flour over the onions and garlic, stirring constantly for 1 to 2 minutes, until no white patches remain.
Gradually add the beef broth to the skillet, stirring continuously to avoid forming lumps.
Stir in the Worcestershire sauce and dried thyme. Season with salt and pepper to taste. Allow the sauce to simmer until it thickens, which should take about 1 to 2 minutes.
Return the pork medallions to the skillet, coating them in the brown gravy onion sauce. Cook for a few minutes to ensure the pork is well-coated and heated through with an internal temperature of 145°F. Enjoy!