Peel and finely slice the garlic and cut the cherry tomatoes in half. Stone the olives and tear into thirds or quarters. Next, add the fennel seeds to a pestle and mortar with a pinch of salt and smash to a powder. Tip into a cup and put aside.
Finely grate the Parmesan. Pick the basil leaves and set aside. Roughly chop the stalks, then add to the pestle and mortar and smash to a paste with a pinch of salt. Mix 2 tablespoons of olive oil, a grating of lemon zest and the juice of half the lemon with the basil paste. Spoon it out onto a plate and place the tuna steaks on top. Turn the tuna over in the dressing, then leave them to marinate for about 20 minutes in the fridge.
Meanwhile, fill and boil the kettle. Put a deep pasta pan on a medium heat and fill with boiling water, add a good few pinches of salt and bring back to the boil. Drop the spaghetti into the water and cook according to packet instructions. Once cooked, scoop out half a mug of cooking water, then drain the pasta.
Meanwhile, put a medium saucepan over a medium heat, then add a splash of olive oil and the garlic. As soon as the garlic is golden, tip in the bashed-up fennel seeds and the cherry tomatoes. Season with a little pepper, cover with a lid and turn the heat down to low. Cook gently for 5 minutes, or until the tomatoes are soft and sweet, then turn the heat off. Tip the drained pasta into the pan with the cherry tomatoes, stir together and season with a little salt, pepper and a squeeze of lemon juice, if needed. Cover with a lid.
Put a griddle pan on a medium heat. Tear the basil leaves into a bowl with the olives. Add the rocket, then dress the salad with a squeeze of juice from the remaining lemon half and a drizzle of extra virgin olive oil. Finely chop and sprinkle over the chilli.
Season the tuna with a little salt, then place in the hot griddle pan and cook for 30 seconds on each side for rare, or for a bit longer if you prefer it more well done. Stir the grated Parmesan into the spaghetti, adding a splash of the reserved cooking water to loosen, if needed. Divide the spaghetti between your plates, lay a tuna steak next to each pile and top with a good pinch of the olive salad.