Heat oil in a skillet over medium heat add mushrooms, onion, carrot, and celery (I threw all of these in my food processor together) and cook, stirring until onion is softened, about 5 minutes. Set aside and cool.
Add all the remaining ingredients (except tomato puree) in a large bowl and mix well. Easiest to mix with just your hands, squish it through your fingers until the mixture is light, fluffy, and uniform in appearance. Be sure to wash your hands before and after touching the mixture with bare hands.
Once the sauteed vegetables are cool, add them to the bowl and mix well. Press meat loaf in loaf pan or form into a loaf on a slotted pan (2-2.5 inches high) so excess fat can drip off (this is what I always do!).
Bake 30 minutes then top with tomato puree or sauce (spaghetti or BBQ sauce tastes far better, but will increase the point value), and bake an additional 30 to 40 minutes. Allow to cool for 5-8 minutes and cut into 8 slices. 2 slices is a serving enjoy!