Drain your beans and place in a large bowl. Drain your tuna and flake that in also. Add your sliced onion. Season with some black pepper.
In a pestle and mortar, smash up your garlic clove with your drained anchovy fillets (you could use less anchovies if you prefer it less punchy but I love them!). Smash and mix until you have a sludgy paste.
In a bowl mix your oil and vinegar to make a dressing. Add in your anchovy & garlic paste and mix it all up.
Pour your dressing over your beans. Add the chopped mint and parsley and mix everything up to coat in the dressing. Serve!