Melt the butter in a large skillet over medium heat. Add the onions, mushrooms, celery and garlic and saute until tender. Add the flour and form a roux, stirring to incorporate the butter and flour.
Slowly add the vegetable stock and stir until smooth. Continue to cook until the sauce coats the back of a spoon. If the sauce looks too thick you can add more stock or water to thin it down.
Add all the blanched vegetables to the sauce along with the fresh herbs. Cook for 5 minutes, gently stirring, over medium heat. Add the mascarpone and heavy cream and allow to reduce for another 5 more minutes. Season with salt and pepper. While the sauce is simmering, add the canola oil to a large skillet, over medium heat, until smoking. Slice the bread into triangles and shallow-fry on both sides until golden brown. Remove from the oil, drain on paper towels and set aside. To serve, place a ladle of vegetable stew into each large soup bowl and garnish with fried toast points.
Cook's Note: Blanching vegetable is a great way to speed up the cooking process of any dish and is very simple to do. Simply cook vegetables in boiling salted water until vegetables are still firm but cooked, then remove from boiling water and place into a bowl of iced water to stop the cooking process. Drain and set aside until ready to use.