Brush a little of the oil very lightly over the tortillas, then drape each one over a small ovenproof bowl or ramekin. Place on a baking sheet and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 5 minutes. Check the tortillas are still centred on the bowls, repositioning them if they have slipped, then cook for a further 5 minutes, or until golden and crisp. Leave to cool.
Rub the remaining oil over the tuna steaks. Press the cumin seeds into the steaks, then season well. Heat a griddle pan until smoking hot, add the tuna and cook for 2–3 minutes on each side until charred but still pink in the centre.
Meanwhile, stone, peel and slice the avocados, then toss together with the tomato, red onion, coriander and a squeeze of lime juice.
Cut the tuna into cubes and pile into the tortilla cups. Top with the avocado mixture, another squeeze of lime juice and a sprinkling of lime rind. Serve immediately.