Clean, wash and roughly chop the shallots and vegetables. Wash and chop thyme. Melt butter in a large pot. Sweat shallots, vegetables and thyme in it while stirring. Dust with flour and briefly sauté lightly. Deglaze with chicken stock and reduce by half for about 30 minutes
Add cream and bring to the boil. Season to taste with salt, pepper and lemon juice. Puree with a hand blender, then pass through a fine sieve. Season again with salt, pepper and lemon juice. Allow the sauce to cool and freeze in portions (e.g. in 250 ml portions)