For horseradish sauce: Mix all ingredients together and let sit for at least 30 minutes.
For fried mushrooms: Heat oil over medium heat. While oil is heating remove stems from mushrooms and dredge in flour.
Beat the egg yolks until thick and lemon colored. In a separate bowl, beat the whites until they stand in peaks. Fold the yolks into the whites. Dip each mushroom in egg mixture until well coated, then place in oil to fry. )
Fry mushrooms until golden brown, flipping and spooning oil over tops when needed. As each mushroom is done, carefully scoop it out with a slotted spoon and place on a plate with a paper towel on it. Serve immediately with horseradish sauce or ranch dressing for dipping.