Blot away the moisture from the lamb pieces with paper towels. Toss the lamb into a bowl, season the pieces with salt and pepper and pour about 2 Tbl of olive oil into the bowl. Mix well. Heat up a 12 inch cast-iron or aluminium skillet over a medium flame. Sear the pieces of lamb in the pan, on both sides, and remove them.
Add 3 Tbl of olive oil to the pan and slightly brown the chopped onions. Throw in the garlic and saute for 30 seconds. Return the lamb to the pan, sprinkle the cayenne pepper over the lamb and onions. Place the pan into the oven for 10 minutes (or until the lamb reaches rare). Remove the pan from the oven and spritze champagne vinegar over the lamb.
Tent the pan with aluminium foil, allowing it to rest for 5 minutes before service.