Pre-heat oven to 350 F (177 C).
In a small bowl, combine 1 tablespoon of ground flax with 3 tablespoons of water (we shall call this flax goo). Allow to sit for at least 5 minutes.
In a medium heat pan, cook finely diced onions for 2-3 minutes until slightly translucent. Add spinach until wilted.
Clean Portobello Mushrooms and remove stem.
In a medium sized bowl, mix together cooked onions/spinach, flax goo, TVP, spices, and feta.
Place the mushrooms on a parchment-line baking sheet, and fill them with the filling.
Sprinkle the vegan Parmesan on top.
Cook for 20 - 25 minutes.