1. Saute onions in a pan with oil until just translucent, add mushrooms and allow to brown (3-5 minutes). Add margarine and garlic and saute for 1 more minute. Set aside to cool.
2. In a large bowl, lightly whisk cream cheese and lemon juice. Add spinach and nutritional yeast, if using and the seasonings.
3. Mix in the mushroom mixture once it is completely cool. Preheat oven to 400 degrees F. Cut each puff pastry sheet into four squares if making regular size, or 8 if small.
4. Place a large spoonful of mixture into the center of each pastry square. Fold into a triangle and seal the edges with a fork.
5. Brush tops with a little milk or oil spray, and bake until golden, about 20 minutes.