In a large pan, sauté onions in heated oil for 5 minutes. Add mushrooms and cook additional 10 minutes, over medium heat. Stir in tomato paste and wine, and cook additional 5 minutes.
Allow to cool, then spoon into 2 large, gallon-sized Ziploc bags. Fill the bags halfway, then seal them well, removing as much air as possible. Lay bags flat on the counter, letting the sauce spread out to fill the entire bag. Freeze the bag flat in a 9x13 inch foil pan. When ready to use, you do not have to defrost the sauce; just cut open the bag, and use it straight from the freezer, as it is the correct size and shape for a 9 x 13 inch foil pan! Yields 2 quantities of sauce.
SERVING SUGGESTIONS: I have used this recipe over a variety of different meats and cuts, including 1st- and 2nd-cut boneless veal; boneless chicken breasts; tenderized minute steaks; and even rib steaks. Enjoy!