Heat oil in a 12-inch skillet over medium-high. Arrange pork medallions in a single layer on a work surface, and press each with the palm of your hand to flatten to an even thickness. Combine salt, garlic powder, and pepper; sprinkle evenly over pork. Add pork to skillet in a single layer; cook just until done, about 3 minutes per side Remove from heat; let stand 5 minutes before serving. Garnish with thyme leaves, if desired.