Make sure you have an oven rack placed in the top position. Preheat the broiler on high.
Remove the spinach from the package and place it in a microwave-safe bowl, covered with wax paper or plastic. Heat the spinach on high for 3 minutes, or until it is completely thawed.
While the spinach is in the microwave, peel the garlic and slice it. In a small pan, heat the olive oil on the stove on medium high and add the garlic. Cook just until it softens a bit, for up to 2 minutes. Be careful not to let it burn. Using a slotted spoon, remove the garlic to a small dish and set it aside.
Take the spinach out of the microwave and drain it in a fine-mesh strainer in the sink. Press the back of a fork into the spinach to squeeze out the liquid. Let the spinach sit while you move on to the next steps.
(You may want to wear food-safe gloves when handling raw bacon and beef for this recipe, or else be sure to wash your hands afterward.) Cook the bacon in a pan on the stove or in the microwave, just until it's cooked through but not crispy. (For the microwave, line a dinner plate with paper towels and add four bacon strips on top, side by side in a single layer. Cover the bacon with one or two paper towels and cook it in the microwave, just until it's no longer raw. Mine takes about 3 minutes and 40 seconds.)
Let the cooked bacon drain on paper towels and make the rest of the bacon, using clean paper towels to line your plate if using the microwave.
While the bacon is in the microwave, you can grate the Parmesan, if needed. (I grate mine in my blender.)
Place the beef on a large cutting board. To tenderize it, use a knife to score the meat, making shallow, diagonal cuts one inch apart. Then make cuts in the opposite direction, making a diamond pattern. (See photo in post.) Flip the steak over and repeat the shallow diamond cuts on the other side.
Loosely cover the meat with plastic wrap (to prevent splashing) and pound the steak with a mallet or the bottom of a pan, working from the center to the edges. Try to pound the steak into a 12x8-inch rectangle. Discard the plastic wrap and add the salt and pepper to the steak.
To assemble the pinwheels, have several toothpicks or skewers set out on the counter. Start with the meat horizontally in front of you. Arrange the bacon slices vertically on top of the steak, in a single layer. Pat the spinach dry with a paper towel and place the spinach over the bacon. Distribute the garlic slices over the spinach and sprinkle on the Parmesan.
Roll up the beef, starting at one of the short ends. Secure the meat closed with the toothpicks placed one inch apart, starting half an inch from one end. Slice the meat between the toothpicks to make the steak pinwheels.
Place the pinwheels on an ungreased broiler pan (or a rimmed baking sheet). Broil three inches from the heat for 3-4 minutes. The steak cooks very quickly, so check on it after three minutes. It will get tough if you cook it too long.
Let the meat rest for a few minutes and serve with balsamic glaze drizzled on top. Store leftovers in the refrigerator for up to four days.
Notes
Top Round Steak: This is sometimes sold as "London broil" and is a lean cut of beef. I prefer it to flank steak here, because top round is a bit thicker and more sturdy for rolling. I used to make flank steak pinwheels, but found the flank steak was so thin, it would tear.Spinach: If you want to use fresh spinach, I would wilt the spinach leaves in a 12-inch pan with a little olive oil when you sauté the garlic. Then, drain and chop up the spinach.Grill DirectionsIf you wish to make these on your outdoor grill, you'll need skewers instead of toothpicks. Thread two pinwheels on each long skewer. Grill uncovered over medium heat, starting with five minutes on the first side and then flipping it over to finish cooking to your liking.