Use a sharp knife or mandolin to very thinly slice the fennel and radish. Toss in the orange segments, remaining juice in oranges, olives, herbs and lemon juice. Season to taste.
Press the BBQ Pepper Spice and a pinch of salt all over the outside of the tuna. Pour a tablespoon of oil in a large heavy-based frying pan on high heat, and place the tuna carefully in the pan, searing it for about 45 seconds (depending on the thickness of the fish) before flipping. You are aiming for a golden crust on the outside of the fish, but for the centre to still be basically raw.
Remove the fish from the pan and slice across the grain. Serve immediately with the fennel salad.
