Heat oven to 350 degrees. Lightly spray a baking sheet with non-stick cooking spray and set aside.
Heat olive oil in a skillet or frying pan over medium heat. Add mushrooms and onion and sauté for 7 or so minutes, until mushrooms have released their water and both vegetables have begun to soften.
Add fresh chard and a tablespoon of water, and continue cooking for 8-10 minutes, until chard softens and becomes tender, stirring occasionally. During that cooking time, you may add another tablespoon or more of water if the chard is sticking to the pan, but you dont want wet chard for this recipe, so be careful about adding too much water. (Reduce the heat and put a lid on the skillet if you find the chard is drying out and sticking to the pan too much.) During the last several minutes of cooking time, add the garlic powder, ground pepper and a light sprinkling of salt. When chard is tender, remove pan from heat, stir in Parmesan cheese and set pan aside to cool.
On a lightly floured board or surface, roll out pizza crust into a 10- by 14-inch rectangle (measure it!). Spread cooled spinach mixture and mozzarella cheese on top of dough, leaving about an inch of plain dough around edges. Starting from long side, roll up the crust to make one large loaf. Pinch long seam to seal, then fold ends under and pinch them to seal. Place loaf onto prepared baking sheet.
Bake for 20 to 25 minutes, until golden brown. Remove from oven, let cool for 5 minutes, then cut and serve.
Note: You may use this as a side dish or appetizer (sliced) or cut in thicker slices and serve with pasta sauce for dipping, to make a meal out of it.