Put all the pepper crust ingredients into a spice grinder, or use a pestle and mortar, until finely ground. Roll the tuna in the ground spices, place into a pre heated frying pan with olive oil, sear on all the sides then place into the oven for 2-3 minutes.
Watch out for the mustard seeds which can jump out of the pan when they start to pop! Remove and allow to rest for a couple of minutes.
Mix all the dressing ingredients together.
Place all the sliced vegetables into a pan with the picked thyme, cook for a couple of minutes just to wilt. Place into a bowl and mix with the spinach leaves, walnuts and some of the dressing - enough to coat all the leaves. The heat of the vegetables will be enough to wilt the spinach leaves slightly. Place the salad mix into the centre of the plate and slice the tuna on top, drizzle with the dressing.