Melt butter in a medium sized pot (non stick), add flour, salt and pepper - as though you were making a rue (blonde rue). Allow flour to melt into the butter and stir constantly until the flour is a blonde rue.
Combine rue with Tomato Paste and slowly add the chicken stock whisking as you go. Once this has come to a moderate heat add marsala and cream - not together or you will cruddle - if you want a dryer taste use a dry sherry instead.
Finally add your star ingredient - Crab and allow to come full heat slowly and serve with a wedge of crusty bread and salad for a full meal or just by itself for a great intro to a wonderful dinner.