Combine equal parts coarse salt and whole caraway seed. Store in clean jar. Heat 1 cup water to a boil. Dissolve cornstarch in 1/2 cup warm water and add to boiled water. Return to a boil and thicken until it coats a spoon. Cool and store in refrigerator.
Preheat oven to 350 degrees F.
To make kummelweck rolls, take 12 rolls and place on baking sheet. Brush top of rolls with cornstarch solution and sprinkle with seed mixture. Place in oven for 4 minutes or until kummelweck dries.
For the Beef on Weck: Heat au jus in saucepan until simmering. Dip sliced roast beef in hot au jus and place on cut kummelweck roll. Top with a dollop of horseradish and dip the top of the roll in the au jus.