1cupmozzarella cheeseshredded, or Monterey jack or white cheddar
1tspeggbeaten with water for egg wash
Place rack in center of oven then preheat to 425 degrees. Line a baking sheet with parchment paper, lightly coat with cooking spray then set aside.
Place a large skillet over medium heat then add 1 Tbsp of butter. Once butter has melted, add chopped spinach then cover and let cook 2 minutes. Uncover and continue cooking until spinach has wilted and become dark green.
Remove from heat then let cool completely. If you are in a big rush, place in the freezer approximately for 5 minutes until eggs are completely cool. Unfold and roll the puff pastry sheet on a lightly floured surface into a 16" x 16" square.
Remove spinach from pan and let cool then press out as much water as possible.
Combine eggs, milk, salt and pepper and whisk until smooth. Wipe out pan and add 1 Tbsp of butter. Scramble egg mixture in pan until just set then add spinach and combine thoroughly.
Place egg mixture in center of puff pastry. Form a 4" wide line running the entire width of the pastry and leave a 1" border along left and right sides. Sprinkle cheese over eggs.
Brush around perimeter of dough with egg wash then fold the top edge of dough all the way up and over the filling. Brush top of dough with egg wash then fold the other side of the dough to overlap the first.
Transfer Stromboli to prepared baking sheet, turning it to ensure the seam is on bottom. Tuck sides under to seal filling inside then brush entire roll with remaining egg wash. Sprinkle with a little kosher salt and pepper then make a few slits in the top with a sharp knife to create air vents. (At this point, you may store the Stromboli in the fridge up to 4 hours before baking.)
Bake 25–30 minutes or until the crust is an even, deep golden brown color. Remove from oven then let rest 5 minutes before slicing and serving.
If you choose to substitute frozen spinach, defrost then squeeze dry before adding to eggs!