First, cook the spinach with a bit of olive oil, salt, pepper, and some garlic powder. Cook until fully wilted, then set aside to cool. Meanwhile, in a small pot with water, boil 3 eggs for about 10-12 minutes, then set aside to cool down.
Finely mince the onion and the garlic cloves, as tiny as possible. You can use a grater or a food processor as well if you prefer.
In a large bowl, add the ground meat, the remaining egg, the grated Parmigiano cheese, the minced onion and garlic, and the breadcrumbs. Season with salt and pepper.
Mix well, preferably with your hands, until all the ingredients are combined.
Roll out a sheet of parchment paper, then spread the meat on top into a square about 9-10 inches (25 cm) and uniformly thick.
Squeeze all the water out of the spinach and spread them in a central layer, leaving about 1 inch (2.5 cm) space on each side. Add the slices of ham on top, then the grated cheese, and finally the 3 peeled hard-boiled eggs.
Grab the parchment paper on one side and start rolling it tightly around the eggs, then roll it on itself leaving the seam side down. If the stuffing starts rolling out, you can grab the other side of the parchment paper and close the meatloaf upwards, by pressing it gently. When done, always roll it over to keep the seam on the bottom.
If needed, move it gently to the middle of the parchment paper, then press the sides with your fingers to close them shut. The ground meat should not break and crumble, but if it does, try to squeeze it together gently (the reason might be that the meat is too dry).
Move the meatloaf to a baking pan by lifting it with the paper. You can choose if you want to cook it with or without the parchment. I tried both ways and didn't notice any difference in terms of cooking time and result.
Add a few slices of butter on top of the meatloaf to keep the meat moist. Bake for 40 minutes in the preheated oven.
There will be some grease in the pan after baking, gently lift the meatloaf up using two spatulas and move it to a serving platter before cutting up the slices.
Can I use lean ground beef? Preferably not. I suggest using a mix of 50% beef and 50% pork for this meatloaf as beef alone would make the meatloaf too dry and crumbly. The fat of the pork will keep the right moisture in the meat for a better end result.