Preheat broiler. For sauce: In a small bowl, combine sour cream, milk, and mustard; set aside. Lightly grease a medium skillet. Half-fill the skillet with water. Bring water to boiling; reduce heat to simmering (bubbles should begin to break the surface of the water). Break one of the eggs into a small dish. Carefully slide egg into simmering water, holding the lip of the dish as close to the water as possible. Repeat with the remaining egg, allowing each egg an equal amount of space.