Shape the vol au vent into 24 pieces using a fluted pastry cutter and glaze 12 of them with egg wash. The remaining 12 using a smaller cutter, cut holes in the middle and place on top of the first 12. Glaze again with egg wash and bake in a 350F oven.
In the meantime prepare the filling by heating a small saute pan with oil and butter.
Add onions, escallion and garlic to sweat for 3-5 minutes.
Add sausages and continue to stir, season with oregano and thyme.
De-glaze with tomato sauce and heavy cream, add cheese until thicken. Set aside and serve.
Fill vol au vent shells with sausage mix, top with mozzarella cheese and herbs, bake until melted cheese has caramelized.