The tuna must be yellow fin and as fresh as possible. It should be bright red and not look dull or discoloured, as this means that the blood has oxidized and it can then taste rather bitter.
Rub a heavy-based frying pan, preferably non-stick, with a little vegetable oil and heat until the oil is almost smoking. Meanwhile, season the tuna with salt and pepper, and sear it for about 30 seconds on all sides until lightly coloured, then refrigerate until required.
If the fennel’s outer leaves are fibrous, peel them. Quarter the bulbs and cut out the cores