Put the beans into a pot of salted boiling water and cook until just done, about 2-3 minutes. Drain and pop them into an ice bath so they stop cooking, then drain and dry them with a dishtowel.
Now make the dressing: put the shallot, vinegar, mustard, olive oil, salt and pepper into a little jar and shake shake shake.
Put the beans, tomatoes and basil into a bowl. Drizzle on the dressing, tasting as you go until the salad is dressing to your liking. Ready to serve!