Wash the fillets in running water, and dry with paper towels. This will remove the strong game-y smell.

Marinade the fillets in plum sauce for at least an hour.

Slice and fry the onions in a little butter until they're turning brown. Remove from the pan.

Sear the outside of the fillets in hot oil and butter 50/50.

Continue cooking on lowered heat, add the onions and cook until the fillets are done medium-rare (no more, - kangaroo can get quite tough).

Remove fillets and set aside to rest on warm plates. Season with freshly ground pepper, there’s probably enough salt from the soy in the plum sauce.

Add the marinade of plum sauce to the pan and make a gravy with the onions and juices.

Plum sauce:
Stew the plums and remove the stones, add soy sauce to taste.