Remove any excess fat and skin and the giblets from the capon

Loosely stuff the capon with the sausage meat, then truss it

Place the capon in a large pot, scatter the giblets, bay leaf, thyme, carrots, and onion around it, then cover it with cold water

Turn the heat to high, bring the water to a boil, then reduce heat to low

Simmer the pot, covered, for 2 hours, adding the celery after 1-1/2 hours. Just before the capon is finished cooking, knead together the butter and flour to make a roux and set it aside

Remove the capon and all the celery from the stock

Puree the celery in a food processor or blender

Put the celery puree and two cups of strained capon stock into a small saucepan and turn heat to low

Bring the puree just to a simmer, then stir in the roux

Simmer, stirring often, for another 5 minutes

Stir in the cream, salt, and pepper

Simmer for 10 more minutes, stirring often. While the sauce is simmering for the last 10 minutes, carve the capon

Stir the lemon juice into the sauce and immediately remove it from the heat

Arrange the capon, sausage stuffing, carrots and onions on a large platter and serve with the sauce in a sauceboat on the side.