Peel the shallots, clean and wash the vegetables and chop everything. Wash and chop the 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 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. Flavour again. Let it cool down. Freeze portion by portion as a base for light sauces