In a frying pan, heat olive oil then saute onions until softened and then add button mushrooms. Saute for about 6 minutes, until most of the liquid has evaporated. Remove from heat and stir in tomato paste. Set aside. Note that This duxelle [minus the paste] can be made ahead of time and will keep for two days in the refrigerator and you can use it to flavor stuffings, potatoes, casseroles, and soups.
In the same pan, heat vegetable oil over high heat adding the portabellos and saute until soft.
Serve the portabello filets with a layer of mushroom filling and hot [1 1/2 cups] demi-glace.
Demi-Glace [reduction sauce]:.
In a large saucepan, heat oil adding all the vegetables and saute until the onions are a golden brown (about 6 minutes).
Add the tomato paste and the garlic. Saute until the tomato paste is well combined with the vegetables.
Add half the red wine and allow it to reduce by one third of the volume. Add the other half of the wine and allow to reduce by one third.
Add the bay leaf, water, and thyme. Bring to a simmer, and allow to reduce by half. Strain the sauce.
Serve hot, or refrigerate until ready to use.
Demi-glace is done when it coats the back of a spoon. If not thick enough, arrowroot dissolved in a little cold water can be added.
[This can be used with other recipes or as a stand alone sauce].