For the tuna, place the olive oil in a saucepan with thyme, garlic, fennel seeds and peppercorns. Very gently heat the oil and just before boiling point – when tiny bubbles start appearing – add in tuna. Ensure tuna is completely covered in oil. If it is not, heat some more oil separately and add it. Turn off the heat and allow to cool for a few hours. Leave the tuna in the oil and place in the fridge overnight.
For the dressing, whisk all ingredients until combined, and season with sea salt to taste.
For the salad, trim and finely slice baby fennel and mix with lemon juice to prevent discolouration. Finely slice the radishes into rounds. Peel and segment the orange.
Lift the tuna from the oil and carefully flake into bite-sized chunks. Mix all salad ingredients together with the dressing and serve immediately.