Chop the onion and garlic into tiny pieces.
Put the turkey breasts on a chopping board, cover with clingfilm or greaseproof paper then beat out into one cm thickness.
In a large dish, mix all ingredients with the onion and garlic, then coat the meat well with this mixture. Leave the meat in this marinade for 1-2 hours.
Heat your grill, barbecue or griddle pan. Shake off the onion bits, season the turkey breasts with salt and cook them for 3-5 minutes on each side. If you use a griddle pan, add the marinade to the pan for the last 5 minutes to make a sauce. If you use a grill or barbecue, then fry this mixture separately in a pan.
Serve warm with the spiced onion salsa over it.