Heat 1 teaspoon of oil in a skillet over medium heat. Add the spinach and cook, stirring, until wilted, about 1–2 minutes; remove and let cool slightly.
In a medium bowl combine the cooked spinach, chopped chicken, eggs, flour, mozzarella, Parmesan, minced garlic, black pepper, chopped green onion, and salt. Mix until evenly combined.
Heat the remaining oil (about 2 tablespoons) in a wide skillet over medium heat.
Scoop about 2 tablespoons of the mixture per fritter into the hot skillet and gently pat each into a flattened round. Cook 2–3 minutes per side until golden and cooked through, flipping carefully.
Work in batches as needed to avoid crowding, transferring cooked fritters to a plate lined with paper towel to drain briefly.
Serve warm with ketchup or mayonnaise if desired.
