Preheat the oven to 200°C (400°F), Gas Mark 6. Put the potatoes into a large saucepan and cover with lightly salted water. Bring to the boil and cook for 15–18 minutes until tender. Drain and keep warm.
Meanwhile, make the spice mix. Place all the spices in a spice grinder or mini chopper and grind to a coarse powder. Sprinkle the spices over a chopping board.
Rub the tuna loin with 1/2 tablespoon of the olive oil, then roll in the spice mix to coat. Heat the remaining oil in a frying pan over a medium heat, then sear the tuna for 8–10 minutes, turning frequently, until browned all over. Place the tuna in a small, nonstick roasting tin and bake in the oven for 10–12 minutes until cooked on the outside. Remove from the oven, cover with foil and set aside to rest for 4–5 minutes.
Heat a ridged griddle pan over a high heat and cook the asparagus, turning occasionally, for 3–4 minutes until just tender and nicely charred. Once cooked, arrange on serving plates with the potatoes. Cut the tuna into slices and serve