Note: Chef Jacobs makes this sauce using the same broth in which his corned beef and vegetables have been cooked. Alternatively, substitute 2 teaspoons of concentrated "Better Than Bouillon" Ham Base (available at most grocery stores) mixed with 2 cups of water.
To make the roux, melt the butter in a small saute pan, then slowly stir in the flour and cook over low heat for 15-30 seconds, stirring constantly. Do not allow roux to brown.
To finish the sauce, bring the broth, cream, horseradish and mustard to a boil in a small saucepan. Reduce heat to a simmer and slowly add the roux, a little at a time until the sauce is thick enough to coat the back of a spoon.
Stir in the chives and serve over corned beef, cabbage and other vegetables.