Preheat oven to 350° F.
Place corned beef in a food processor and chop finely, remove. Add onion and celery and finely chop.
In a bowl combine egg, rice and mustard. Mix in corned beef, onion and celery. Place 1/2 cup of mixture on each cabbage leaf and roll up sticking in ends. Place seam-side down on 13 x 9-inch baking pan.
Dissolve bouillon cube in boiling water, add beer and pour over cabbage rolls. Cover tightly and bake 1 1/2 hours.
Remove from oven and reserve 1 cup of liquid.
Melt better in saucepan, add flour, stir and cook over low heat 1 minute. Add reserved 1 cup of liquid, increase heat to medium high and bring to a boil. Reduce heat and simmer until thickened. Pour over cabbage rolls and serve with additional mustard.
Servings: 10