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Boil or steam the green beans and carrots for 8-10 mins until just tender. Put the sliced onions in a bowl and pour over boiling water until just covered.
Meanwhile, make the dressing. Mix all the ingredients together in a large bowl.
Tip the cooked beans and carrots into the dressing along with the drained onions, canned beans and tomatoes, toss well, then add the basil and toss again. Serve scattered with the mozzarella and a grinding of black pepper, if you like. Will keep chilled for up to three days.