Use a sharp knife to make small incisions in the tuna steaks. Insert some garlic, coriander seeds and mint into each opening.
Make the sauce. Heat a little of the oil in a saucepan, add the garlic, chilli, oregano and any coriander seeds remaining from the tuna. Cook, stirring, until the garlic turns golden brown. Add the mint, wine and tomatoes and cook over a medium heat for 5–10 minutes.
Heat the remaining oil in a small flameproof casserole on the hob. Add the tuna and seal on both sides. Pour the sauce over the tuna, season to taste with salt and pepper and transfer the casserole to a preheated oven, 220°C (425°F), Gas Mark 7, and cook for 15–20 minutes.
Sprinkle the baked tuna with a little mint and the capers. Serve with new potatoes and steamed spinach, spring greens or broccoli.