In a pan, melt butter. Add flour and salt. Cook, stirring frequently, until the mixture becomes pale golden with a slightly nutty aroma.
Now pour warm milk gradually, stirring constantly with wooden spoon and whisk until smooth. Cook this mixture, stirring constantly along the bottom of the pan. Cook till the sauce thickens. This takes about 4-5 mins on med to low flame.
Once the sauce is thick enough, add the chopped spinach and give it a good mix.
Next add in the crumbled paneer.
Also add the seasonings - salt, dried parsley, onion powder, garlic powder and red pepper flakes. Cook for another 2-3 minutes.
The sauce finally should be thick enough to spread and not drip. Take it off the flame and let it cool down a bit. Taste a add salt if needed.
Pre-heat your oven to 400°F(200°C). Then, thaw the frozen puff pastry sheets. Cut the sheets into 5-inch squares and place them on a cookie tray with parchment paper.
Add a generous spoonful of the spinach - paneer mixture in the center.
Top with mozzarella cheese.
Fold the sheet from all sides. You need not close it completely. Spray cooking oil or brush milk on top. Place them in the oven and bake for 20-25 mins or until they brown on top.
Spinach Paneer Puffs are ready. Enjoy them as is or with any hot sauce.