1. Mix together the yeast and warm water and let it sit for 10 minutes.
2. Place the 3 cups of flour, butter, egg yolk, salt and sugar in a mixing bowl (or food processor, if you are using) and add the yeast.
3. Knead until you have smooth dough, adding some milk as needed.
4. Cover the dough with plastic wrap and leave it in a warm place for 1-2 hours to proof.
5. Divide the panzerotti dough to 4 balls and roll into 7-inch circles.
Spinach and ricotta filling:
6. Saute the spinach, red pepper and onion in little butter, and season to taste with salt, pepper and nutmeg.
7. Remove from heat and stir in the ricotta cheese and egg white.
8. Share the spinach and ricotta filling between the four pizza dough circles, placing it only on one half of each circle.
9. Sprinkle with mozzarella cheese over the filling and fold the other half of the dough on top of the filling and make sure you close the ends properly.
10. Help yourself with a fork, if needed.
11. Deep-fry the panzerotti until golden, turning it once to get cooked on both sides.
12. Alternatively, you can brush the panzerotti with egg wash and bake it in the oven.