Drain and rinse the cannellini beans and place in a large bowl then drain the tuna and add to the bowl.Finely chop the red onion and add to the bowl with the cannellini beans and tuna. Grate the zest of 1 lemon over the tuna and beans then squeeze over the juice of half the lemon. Season with salt and pepper.Finely chop the basil and add to the bowl along with 2-3 tbsp olive oil (add more if desired). Stir everything until combined. Enjoy.