Cook the onion: Heat a large skillet over medium heat. Add the chopped onion and cook uncovered for 10–12 minutes, stirring occasionally, until soft, translucent, and lightly golden.

Toast the breadcrumbs: Add the vegan butter to the skillet and stir until melted. Add the panko breadcrumbs, reduce heat to low, and cook for 2 minutes, stirring to lightly toast. Remove from heat and let cool slightly.

Prep the oven & spinach: While the onion mixture cools, preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.
Thaw the spinach, squeeze out as much liquid as possible, then roughly chop.

Blend the mixture: Add the chopped spinach, cooled onion–breadcrumb mixture, flaxseed, vegetable broth (or water), vegan parmesan, nutritional yeast, salt, and spices to a food processor. Also add optional vegan feta if using.  Pulse until combined and slightly sticky, scraping down the sides as needed.

Adjust if needed: The mixture should hold together when pressed. If it feels too dry, pulse in 1 tablespoon of broth or water.

Form the balls: Scoop approximately 2 tablespoons of the mixture and press firmly to shape into balls. Use a cookie scoop to make this step easier. Place on the prepared baking sheet. 

Bake: Bake for 20 minutes, until set and lightly golden. Let cool on the baking sheet for a few minutes before serving.