In a small bowl, combine all of your gochujang sauce ingredients until silky and smooth.
In a large bowl, combine your chopped and wilted spinach, crumbled feta, grated cheddar cheese, and your gochujang sauce.
Get a filo pastry sheet and lay it on a clean surface with the shortest side nearest to you. Brush one half of the sheet with your melted butter, then fold it in half to create a long rectangle (see my Instagram video if you're lost here). Brush the tip of the pastry with more butter.
Place a quarter of your gochujang feta mixture at the bottom right-hand corner of your pastry. Then, gently fold in a triangular motion all the way to the top of your pastry. Seal and set aside. Repeat this step for all 4 pastries.
Brush the top of your pastries with more butter and then sprinkle with sesame seeds. Place in the air fryer for 20 minutes at 160C. Or, place them in a preheated oven at 180/160C fan for around 20 minutes. Remove from the heat and enjoy!