First we need to make the filling sauce. 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 10 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 and the seasonings. Cook for another 2-3 minutes. The sauce finally should be thick enough to spread and not drip.
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 spoonful of the spinach - paneer mixture in the center. Cut a mozzarella stick half-inch size and place each on top of the mixture. Fold the sheet from all sides. You need not close it completely.
Brush some egg + water mixture over all of them. Sprinkle Parmesan cheese over it. Place them in the oven and bake for 20 mins.
Spinach & Paneer Puffs are ready. Enjoy them as is or with any hot sauce.
To make it eggless, rub some melted butter or maida + water over the puffs before baking. You can make the pastry sheets at home as well. Try a variation with any other filling.