Defrost frozen spinach and squeeze out excess water. Set aside.
Heat olive oil in a medium skillet add onions and sauté until soft.
Add the mushrooms and cook, stirring, until they have released their water and it evaporates. Add salt and pepper, to taste.
Transfer to a bowl and allow to cool slightly. Add the spinach, breadcrumbs, Parmigiano, parsley, and egg. Mix well.
Prepare the meat mixture:
Preheat oven to 350F and cover a rectangular sheet pan with parchment paper. Prepare a second sheet of parchment paper of equal size.
In a large bowl combine the ground meat trio, bread crumbs, mozzarella, egg, Worcestershire sauce and salt and pepper. Mix well.
Spread meat mixture onto a rectangular baking sheet covered with parchment paper. Cover with another piece of parchment paper and flatten, using a rolling pin to completely cover the sheet pan.
Remove top parchment paper and set aside. Spread the spinach mixture evenly over meat leaving a 1 inch border all around.
Roll up meat from one of the long ends jellyroll style, lifting with the paper as you roll. Place the meatloaf on a sheet of parchment paper on the baking sheet.
Bake for about 1 hour or until a meat thermometer registers 170F. Let sit a few minutes before slicing it. Serve hot.
When cooking with ground meat, I prefer using the combined trio of beef, veal and pork. However this recipe will work with well with any combination of different ground meats or you can stick to one variety only.
Prepare the spinach and mushroom mixture ahead of time, which is ideal as it needs to cool.
This recipe must be assembled right before baking. I noticed that if you prepare it hours prior to baking, the layer of meat touching the spinach mixture remains red and has an undesirable texture.
Let the meatloaf sit a few minutes before slicing into it. It will hold it's shape better.