Preheat oven to 190 degrees celsius (fan-forced). Grease 3 flat baking trays with cooking oil spray and set aside.
Place the defrosted spinach into a strainer and squeeze out all of the liquid using paper towel (see notes).
Place the onion into the Thermomix bowl and bowl on Speed 6, 5 seconds. Scrape and repeat twice more (until it's finely chopped).
Add the drained spinach, crumbled feta, ricotta, egg, garlic and salt and pepper to the Thermomix bowl. Mix on Reverse, Speed 4, 10 seconds. Scrape down the sides and repeat until the mixture is completely combined.
Cut each pastry sheet into 6 equal sized rectangles (so you will have 36 rectangles in total).
Divide the filling between 18 of the pastry rectangles (leaving the remaining 18 without filling). Ensure you place the filling into the middle of each pastry rectangle, leaving a small gap around the entire outside edge.
Brush egg over the edges of the pastry rectangles that have the filling.
Place a pastry rectangle (with no filling) onto the top of one of the filling pastry rectangles. Use a fork to join the two pastry sheets together into one rectangle. Repeat with the remaining pastry sheets.
Use a knife to cut 2 small slits through the top pastry sheet.
Brush over the egg and sprinkle with sesame seeds.
Place onto the prepared trays and bake for 25 minutes or until golden.
In order for your pastry to go crispy when cooked, ALL of the liquid must be strained from the spinach. I find that it's easiest to place it into a strainer and use paper towels to really squeeze out the liquid.