Preheat oven to 365. In a small saucepan, heat the milk on medium until scalded. Stir in cornstarch until mixture is very thick. Remove from heat. In a saut pan, melt butter over medium heat.
Add onions add cook until softened.
Add mushrooms and continue to cook until softened. Add white wine and let the mixture cook until wine has been reduced to about 2 T. of liquid. Salt and pepper to taste and add teaspoon of rubbed sage. Stir egg yolks in a large mixing bowl. Add a spoonful of the milk mixture to the eggs, while stirring continuously, so as not to cook the eggs.