To make the dressing, combine the pepper, mustard powder and grated onion in a mortar to form a paste. Transfer to a mixing bowl. Combine the sesame and vegetable oils together with the vinegar and soy. Add to the paste and whisk together.
Put the palm sugar in a small saucepan with a dash of water and caramelise until a golden brown. Add 25ml of water to stop the caramelisation. Slowly add the caramelised sugar to the dressing. Mix the wasabi paste with two tablespoons of the dressing and whisk until smooth. Add it to the dressing and whisk together.
Slice the tuna into ½cm-thick slices and arrange over four plates. Dress liberally with the sashimi dressing. Garnish with baby cress, the sliced spring onion, chives, matchsticks of pickled ginger and sesame seeds. Grate the fresh wasabi very fine, shape it into a ball and serve beside the tuna.
Notes: This recipe is provided care of this sponsor and is not an official MasterChef Australia recipe.