Preheat the oven to 350°F. Place steak in a baking dish coated with cooking spray.
Butterfly your steak by slicing them in half with a very sharp knife. Don't cut completely through by leaving one edge uncut. Season each side with salt and pepper.
Stuffing
Heat a non-stick skillet with oil on medium heat. Add onion, rosemary, oregano, and garlic. Cook until onions are translucent. Remove from heat and allow to cool down.
In the same skillet, add soy sauce, spinach, and breadcrumbs with onion mixture. Stir until well combined.
By leaving 1 inch on the edges, spread butterflied steak evenly with the stuffing. Spread mozzarella on top. Roll the steak tightly. Use cooking twine to secure the steak.
Grease a large enough baking dish with butter. Place rolled steaks in the baking dish.
Brussels sprouts.
In a large bowl, mix Brussels sprouts, salt, and pepper. Transfer them all around the rolled steaks. Dizzle Brussels sprouts and steaks with oil.
Bake, uncovered, until meat is tender, 40 to 45 minutes.
Remove stuffed flank steaks from the oven and let rest for 15 minutes before serving.
Store leftovers in an airtight container for 2 days in fridge.