Gather all ingredients and equipment.Preheat oven to 350°F (180°C).
In a food processor or blender, chop mushrooms into small pieces. Set aside.
In a sauté pan, heat up olive oil and sauté diced onion for about 3-4 minutes until golden brown. Stir chopped mushrooms.
Sauté mushrooms for about 5 minutes and let the mixture cool slightly.
In a large bowl, combine remaining ingredients and mushroom mixture. Make sure that all of the ingredients are evenly mixed.
Begin shaping the meat mixture into meatballs, about 1-1/2 inches thick. Once rolled, place the meatballs on a large sheet tray.
Place a large skillet over medium-high heat and spray with oil to coat the pan. Once the pan is heated, add the meatballs one at a time and cook until golden brown on each side. Continue to do this until all of the meatballs have been seared to a golden brown. Once seared, place on a sheet pan.
Place the sheet pan in the oven for approximately 10-15 minutes or until the meatballs are fully cooked through. Remove the meatballs from the oven and serve.
