Prepare marinade a few hours ahead.
Heat 2 tablespoons oil and add aromatics and cook for 30 seconds.
Add sugar, mirin, soy sauce and wine and simmer for 1 – 2 minutes.
Remove from heat and cool.
Place beef in a dish and pour over marinade.
Cover and leave for 3-4 hours, turning occasionally.
Preheat oven to 220degC.
Remove meat from marinade, wipe dry.
Heat oil in an ovenproof pan and sear meat all over. Place in oven for 15 minutes, reduce heat to 180degC and continue roasting (25 minutes for rare; 35 minutes for medium; 45 minutes for well-done).
Remove and cover with foil for 10-15 minutes.
In a saucepan simmer marinade, stock and chestnuts until tender. Remove chestnuts and keep warm.
Strain remaining liquid and return to pan along with any beef juices.
Bring to boil, add the mustard and boil hard for 5–10 minutes. Strain.
Slice the beef and spoon over the sauce and serve with mashed sweet potato and wilted spinach.
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