Preheat your oven to 375°F (190°C).
Lay out the thinly sliced beef on a cutting board. Season with salt and pepper.
Layer spinach leaves on top of the beef slices. Sprinkle mozzarella cheese over the spinach.
Starting from one end, roll up the beef tightly to create a pinwheel shape.
Secure the pinwheels with toothpicks to hold them together.
Heat some olive oil in a skillet over medium-high heat. Sear the beef pinwheels on all sides until browned, about 2-3 minutes per side.
Transfer the pinwheels to a baking dish and bake in the preheated oven for about 10-15 minutes, or until the beef is cooked to your desired doneness and the cheese is melted.
Remove the toothpicks before serving. Enjoy!