Thinly slice the onion and cook gently in butter until soft and brown.If the mixture is too thick add a little vegetable broth. Puree the mixture and sieve.
Add cream, egg white, sugar and salt and pepper to taste. Put into a 500 g siphon with a gas cartrige and keep warm in a bain-marie.
Crisp bacon
Put the slices of bacon in a non-stick pan over a low heat with a weight on top. Fry until the slices crisp. Drain on absorbent paper and crush.
To finish
Siphon the hot onion mixture into cappuccino cups three quarters full. Sprinkle with the crushed crisped bacon and garnish each cup with 5 drops of balsamic. Serve with toasted brioche bread.