For the broccoli basil mousse, clean the broccoli, wash and divide into small florets, peel the stalk and dice finely. Pluck basil leaves from stems and set aside. Place basil stalks with broccoli and onion in perforated cooking container and cook (10 minutes at 100°C in unperforated container or 4 minutes at 120°C in perforated container).
Meanwhile, soak gelatin in plenty of cold water. Puree broccoli, onion and basil while still hot and season with salt, pepper and lemon juice. Squeeze gelatin, dissolve in warm mixture and let cool.
Finely chop basil leaves. Whip cream until stiff and fold in basil as soon as broccoli mixture begins to gelatinize. Refrigerate for at least 4 hours or overnight.
Before serving, clean tomatoes, cut into thin slices and arrange on plates. Clean spring onion, cut into fine rings and sprinkle on top. Mix vinegar, oil, salt and pepper, drizzle over tomatoes and onion rings. Make dumplings of the mousse and arrange on top.