1. Make the tuna marinade

• Pour a little olive oil into a shallow dish. Add the lemon juice, ground cumin, crushed garlic, salt and black pepper, then stir until combined.
• Place the tuna steaks in the marinade and turn them carefully to coat both sides.
• Cover and refrigerate for about 15 minutes while you prepare the vegetables. Avoid marinating the tuna for several hours because the lemon juice can begin to change and firm the texture of the fish.

2. Prepare the potatoes

• Peel and rinse the potatoes, then cut them into thin, evenly sized rounds.
• Thin slices are important because potatoes take longer to cook than tuna.

If your potato slices are thick or you are using a firm, waxy variety, steam or parboil them for 8 to 10 minutes. They should begin to soften but must still hold their shape.

3. Prepare the remaining vegetables

• Peel the onions and cut them into rings. Wash the tomatoes and slice them into rounds.
• Keep the tomato slices slightly thicker than the potatoes so they do not completely break down during baking.
• Prepare the garlic, thyme, bay leaves and chopped parsley so they are ready for assembling the dish.

4. Soften the onions

• Preheat the oven to 200°C/400°F.
• Pour a thin layer of olive oil into a tagine, casserole or large oven-safe sauté pan.
• Place it over low heat, add the onions and cook for a few minutes, stirring occasionally.
• Remove them from the heat when they are beginning to soften and look translucent. Do not allow them to burn or become deeply browned.

If your baking dish cannot be placed on the hob, soften the onions in a frying pan before transferring them to the dish.

5. Assemble the vegetable layers

• Spread the softened onions evenly over the bottom of the baking dish.
• Arrange the potato slices over the onions, overlapping them slightly if necessary.
• Add the bay leaves, followed by the tomato slices. Season with salt, black pepper, thyme and a little crushed garlic.
• Pour a small amount of water down the side of the dish. Add only enough to create steam and a light sauce while the vegetables cook; they should not be submerged.

6. Begin baking the vegetables

• Cover the dish and bake the vegetables for 15 to 20 minutes.
• Carefully test a potato slice with the tip of a knife. It should have started to soften but does not need to be completely tender. If it still feels completely firm, cover the dish and continue baking for another 5 minutes before adding the tuna.
• Beginning the potatoes first prevents the tuna from having to remain in the oven for the entire vegetable cooking time.

7. Add the marinated tuna steaks

• Remove the dish from the oven. Arrange the tuna steaks in a single layer over the partially cooked vegetables.
• Add a small splash of water to the marinade remaining in the dish and stir. Pour it around the tuna and over the vegetables rather than washing all the seasoning off the surface of the fish.
• Add the lemon slices and olives, if using. Sprinkle over half of the chopped parsley, reserving the rest for serving.

8. Finish baking the tuna

• Cover the dish again and return it to the oven. Bake for approximately 12 to 15 minutes, depending on the thickness of the tuna steaks and your preferred level of doneness.
• Start checking thinner steaks early. The potatoes should be tender when pierced with a knife, while the tuna should be cooked to your liking without becoming dry.
• Avoid extending the cooking time simply to brown the top. Tuna is a firm, lean fish and can quickly lose its moisture when overbaked.

9. Rest the dish

• Remove the dish from the oven and leave it covered for about 5 minutes. This short resting time allows the cooking juices to settle and the remaining heat to finish cooking the fish gently.

Do not leave the dish covered for too long, as the tuna will continue cooking in the residual heat.

10. Garnish and serve

• Remove the bay leaves and sprinkle the remaining fresh parsley over the tuna and vegetables.
• Serve one tuna steak per person with potatoes, tomatoes, onions, olives and some of the lemony cooking juices. The dish can be served on its own or with a green salad and warm crusty bread.