For meatballs: Line a large baking sheet with foil and spray with nonstick cooking spray. In a large bowl, combine all ingredients for meatballs. Mix thoroughly with your hands until all ingredients are well incorporated. Form mixture into about 25 (1-1/2-inch) meatballs. Place on the lined baking sheet; set aside.
For cooking: Set a large nonstick skillet set to medium high heat; add 1 cup olive oil. Once oil is hot, add meatballs and fry until golden brown, 5-7 minutes, flipping once halfway through. (If meatballs appear undercooked, place on lined baking sheet and into a 350-degree F preheated oven for 10-15 minutes). Remove with a slotted spoon to a paper towel-lined plate. If necessary, fry in batches.
Carefully discard oil and scrape browned bits out of pan. Reduce heat to medium-high. Add 1 Tbsp. oil. Once hot, add mushrooms and minced onion to pan, cooking until softened and browned. Add Marsala and reduce liquid by half, about 1 minute.
Return meatballs back to the pan and toss in sauce until well coated.