Bring a pot of salted water to a boil. Add the spinach and cook for 1 minute. Drain, rinse under cold water, then squeeze dry, finely chop and set aside.
In a large bowl, combine the eggs, Parmigiano-Reggiano, milk, pepper and salt. Whisk until mixed. Add the chopped spinach and whisk.
Melt 1 tablespoon of the butter in an 8-inch nonstick skillet over medium-low heat. Pour in 1/3 of the egg batter, cover the pan, and cook until the edges of the frittata brown slightly and the top is firm and not runny, 5 to 7 minutes. Transfer the frittata to a plate to cool. Repeat this step until you have 3 small frittatas.
Place the beef cutlets between two sheets of plastic wrap and pound to 1/8 of an inch, making sure not to punch holes in the cutlets. The meat should match the size of the frittata.
Top the beef with the frittata. Roll the 2 together and secure with twine, making sure not to tighten too much, otherwise the twine will break through the beef, or push the frittata out of the roll. Repeat with the remaining beef and frittatas.
Heat the olive oil in a large skillet over medium-high heat, add the garlic and cook until it starts browning, 2 to 3 minutes. Add the rolls, searing the meat evenly all over by rolling them in the pan for 4 to 5 minutes.
Add the wine and cook until the scent of alcohol disappears, 3 to 4 minutes. Reduce the heat to medium-low and cook, partially covered, until the rolls are firm, 15 minutes.
Transfer the rolls to a cutting board to rest for 5 minutes. Slice 1/4-inch thick and serve with a drizzle of the pan juices.