Peel and top and tail the onion. Cut it in half through the root and top and place the cut side flat on your board. Run your knife from the top to the root, cutting with the grain of the onion, creating slices roughly 5mm thick. Fill a small bowl with iced water and stir in 1 teaspoon of salt. Separate the slices into individual slivers of onion and submerge them in the icy cold water for 10 minutes.

Empty the beans into a fine-mesh sieve or colander and rinse under cold running water. Cut the roasted peppers into strips and the tomatoes into a chunky dice. Tip into a bowl along with the beans and tear in the black olives. Finely chop the parsley and add to the bowl.

Drain and add the onions and tear in the basil. Add the zest and juice of the lemon, the vinegar, capers and a generous glug of olive oil. Season generously with salt and about 12 twists of black pepper then toss the salad together. Allow to the salad to sit for a minute or two before you have a taste, just to let the ingredients react to the seasoning. Adjust the seasoning as you like then tip in the drained tuna. Toss two or three times to lightly break up the fish and incorporate it into the salad. Divide between plates and top with extra olive oil and a sprinkle of sourdough crumbs.