Preheat convection oven to 350 F.
In food processor, place mushrooms. Process until finely minced.
On parchment-lined sheet pan(s) coated with cooking spray, place mushrooms
and onion in single layer. Coat with cooking spray.
Bake until mushrooms are browned, 15 to 20 minutes. Cool.
Reduce convection oven to 300 F.
In mixer bowl, combine mushroom mixture and remaining ingredients. Mix on low speed until well combined, 3 to 4 minutes. Do not overmix.
On parchment-lined sheet pan(s), using medium-sized scoop (look for #30 or scoop that portions ~2 Tbsp) portion meatballs 1" apart.
Bake until internal temperature reaches 165 F, 16 to 18 minutes. Drain.