Place kumara into a saucepan with the garlic clove, cover with water and bring to the boil; cook 10 minutes or until tender.
Drain and combine with stock and 2 tablespoons of oil; Mash until smooth and season well with salt and pepper.
Preheat a char-grill or frying pan over medium high heat and brush steaks with remaining oil and season with salt and pepper. Cook 2-3 minutes each side or until cooked to your liking; remove and rest, loosely covered with foil.
To make mustard sauce: Add garlic, stock, water, mustard and sugar to the same frying pan.
Bring to the boil over high heat. Boil for 7 minutes or until reduced by two-thirds.
Whisk cornflour with a 1 teaspoon of water to form a paste. Stir into sauce and bring to the boil until thickened; season with salt and pepper.
To serve, place mash onto centre of plate, top with steak and drizzle over mustard sauce and serve with steamed green vegetables.
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