Heat the oil in a large, non-stick skillet over medium heat. Add the onions, pepper (optional), salt and black pepper and cook until soft -- 5-7 minutes. Remove from heat and let cool.
Preheat oven to 350°F.
Line 2 baking sheets with parchment paper.
Squeeze the spinach to remove all the liquid. Add to food processor.
Add the onion/pepper mixture, ½ cup of the bread crumbs, the Gouda, Parmesan, egg, butter, and nutmeg (if using).
Process until completely combined and smooth.
Remove from the food processor and roll into 1-inch spheres. Roll each sphere in the remaining bread crumbs to coat completely. Place on parchment-lined baking sheets.
Bake until lightly golden, 8-10 minutes. Do not overbake as they will dry out.