Blanch the tomatoes, quench, peel, quarter, core and cut into small cubes. Finely dice the onion. Mix the vinegar, salt, pepper, a pinch of sugar and olive oil into a vinaigrette. Stir in the diced tomatoes, onion and pistachio nuts. Refrigerate.
Cut the beetroot and drained mozzarella into thin slices. Alternate beetroot, mozzarella and basil leaves on a plate. Spread the tomato vinaigrette on top and serve.