Set a medium saucepan over medium heat and melt the butter. Sprinkle the flour over the melted butter and whisk them together to form a paste. Reduce the heat to low, then pour in the chicken stock and heavy cream, stirring continuously to form a thick gravy.
Remove the cream sauce from the heat and transfer it to a large bowl. Stir in the sage, thyme, salt, and black pepper. Cover the bowl with a linen towel and set it aside.
The vegetables need to be slightly cooked before being baked in the pie, or they will be too crunchy. Heat 1 tablespoon (15 ml) of the oil in a medium skillet over medium heat. Add the onion. Saute the onion for 2 to 3 minutes, until it becomes soft and translucent. Add the remaining 3 tablespoons (45 ml) of oil. Add the garlic, carrots, and green beans to the skillet and cook the mixture for about 2 minutes. Add the mushrooms and cook the mixture for 3 minutes.
Remove the vegetables from the heat and transfer them to the cream sauce. Stir the filling with a rubber spatula to distribute the vegetables evenly throughout the sauce. Add the chicken and mix the ingredients again. Let the filling cool for about 5 minutes while you prepare the topping.
To make the topping, place the breadcrumbs, butter, oregano, sage, thyme, and paprika in a small bowl. Stir the ingredients with a fork to combine them, and then mix in the Robusto Gouda cheese.