Pour beans into a sieve, rinse with cold water, drain well. Clean the spring onions and cut into rings. Wash tomatoes, cut in half. Finely dice the onions. Mix everything well.
For the vinaigrette, chop the garlic very finely, mix with the mustard, vinegar, salt, pepper, sugar and 75 ml of cold water. Withhold oil. Pour the vinaigrette over the lettuce, pluck the basil leaves from the stems and sprinkle over the lettuce.