In a deep saucepan, melt butter and saute onion and garlic until golden. Slice tomatoes (if using canned tomatoes, drain first) and add with dry mustard, salt, and pepper. Simmer, uncovered, 8 - 10 minutes.
Stir in tomato paste. (If tomatoes are not full-flavored, you may wish to add more paste.) Add flour and chicken broth, then bring to a boil, stirring constantly. Puree in a blender, then strain to remove seeds and skins. Add 1 - 2 cups light cream.
Reheat to serve, but do not boil. add sherry to taste. This soup keeps well and may be frozen.