Cut onion into small cubes and sauté in butter. Add flour and fry briefly. Add white wine, bouillon and whipping cream and simmer for 15 minutes. Boil washed arugula in salted water for half a minute, remove and quench with ice water. Drain the arugula well, squeeze it out and then puree it finely with a hand blender. Add the arugula puree to the soup, bring to the boil again briefly and season to taste with salt, pepper and a dash of Aceto Balsamico bianco.