Prepare mushrooms by removing stems, wiping clean (or giving a super quick rinse under water and drying with paper towels if really dirty). Dry completely.
Using a knife, not a food processor, chop mushrooms into a fine dice. No large pieces.
In a skillet on medium-high heat, add the oil and chopped mushrooms.
Season immediately with salt and pepper. The salt will draw out the moisture in the mushrooms. Continue to cook until all moisture has evaporated.
Meanwhile, dice the onions finely.
In a separate saute pan, add the vegan margarine and the onions. You can reduce the heat to medium at this point. Cook, stirring frequently, until onions are tender and just starting to caramelize. They should be soft and moist, golden in color, but not crisp. You can add additional margarine if necessary.
NOTE: You could add the onions directly to the mushroom mixture to cook, but I really feel like the separate cooking time allowing them to caramelize adds to the depth of flavor of the finished meatball.
Add onion mixture to mushroom mixture, which should now be golden brown.
Remove from the heat and stir in the minced garlic. It will cook a little from the residual heat of the pan.
Prepare the flax "eggs" by combining the ground flax with water and allowing to sit in the refrigerator. It will become gelatinous and be our binder for the meatballs.
Transfer to a mixing bowl and allow to cool slightly.
Add the oats, vegan parmesan, bread crumbs parsley and flax mixture. Combine with a fork until coarse and crumbly.
Add the mustard and worcestershire.
Season with salt and pepper. Add oregano if using for pasta and sauce. Omit if using for an appetizer, such as sweet and sour meatballs.
At this point the mixture should hold together. If it seems a bit dry, add another Tablespoon of water (or you could add a splash of almond or coconut milk).
Cover with plastic wrap and refrigerate for several hours. Preferably overnight. This will allow the moisture to permeate the dry ingredients and the flavors to meld.
Preheat oven to 425 degrees.
Using a small scoop, form uniform balls onto a lined sheet pan. Using moist hands, roll into smooth meatballs.
Bake for 10-15 minutes. They should be slightly golden but not browned.
If using for pasta sauce, this would be the point where I would finish cooking them in the sauce.
If using for appetizer meatballs, finish cooking them in the sweet and sour sauce.
Since we are not using eggs or raw meat here, they just need to be heated through and hold their form.