Preheat oven to 400 degree F. Grease a baking sheet; set aside. In a small bowl stir together Boursin cheese and the 1/4 cup green onions; set aside.
Unwrap biscuits. Using your fingers, gently split the biscuits horizontally. Place the biscuit bottoms on prepared baking sheet. Spread about 1 tablespoon of the cheese mixture over each biscuit bottom. Replace biscuit tops.
In a small bowl use a fork to beat together egg yolk and the water. Brush biscuit tops with egg yolk mixture. Sprinkle with Parmesan cheese and additional sliced green onions. Bake in preheated oven for 8 to 10 minutes or until golden. Serve warm. Makes 10 biscuits.