Generously season the lamb medallions with kosher salt and pepper on both sides, and heat the oil in a large skillet. When the oil is hot but not smoking, place the medallions in the pan and cook for 2–3 minutes on each side.
Add a little more oil to the same pan and heat over med/high heat. Add the onion and cook until translucent, about 5 mins. Stir in the garlic and ginger and cook for 2 more mins.
Stir in the chicken stock, coconut milk, brown sugar and red pepper flakes. Bring to a boil and reduce to simmer. Cook until the sauce thickens slightly, about 4 mins.