Tear the bread into large chunks – leaving out the crusts. Spread them on a baking tray lined with baking parchment, toss with a little oil and season with salt.
Bake the croutons in a preheated conventional oven (top and bottom heat) at 200°C for about 18-20 minutes, then leave them to cool.
Tuna sauce
Put all the ingredients in a mini chopper or similar and blend until you have a smooth, even cream. Season to taste with salt and freshly ground pepper - and a little more lemon if needed. Adjust the consistency too, with more olive oil or a splash of water.
To serve
Pat the tomato slices dry with a little kitchen paper so they don't release too much liquid. Divide them between 4 plates.
Spoon the tuna sauce over the tomatoes, ideally in small dollops that you spread into swirls with the back of a spoon, then scatter the croutons on top.
Sprinkle with capers and fresh basil leaves, add a little olive oil if you like, and season with freshly ground pepper and a pinch of flaky salt.