Prepare the instant couscous according to the package directions and let cool. Finely grate the lemon zest and squeeze 3 tbsp of juice. Mix 3 tbsp oil, lemon juice and zest with the couscous and season with salt and pepper.
Peel the mango, remove core and dice. Dice 100g cornichons, mix with the diced mango and 2 tbsp of the cornichon brine and season with salt and pepper.
Slice the remaining cornichons and add them with another 4 tbsp of the brine to the couscous.
Mix the fennel seeds with the remaining oil, salt and pepper and use it to brush the tuna with. Grill on both sides for about 10 minutes on a barbecue over low heat.
Cut the tune into quarters and serve with couscous and cornichons and mango relish.
