Preheat your oven to 350 degrees.
Add olive oil and butter to a medium-sized pan over medium-high heat. Once both melt together, add mushrooms, onions, rosemary, and salt and pepper to taste, stir and cook until tender.
Next, add the minced garlic and cook for an additional minute making sure not to burn it. Add in Worcestershire sauce, stir together, and cook for an additional 1-2 minutes. Then, set the mixture aside while you prepare the steak.
Lay butterflied steak on a cutting board and season with salt and pepper on the outside of the steak then turn over. Spoon the mushroom mixture in an even layer across the steak.
Next, arrange spinach and sprinkle breadcrumbs evenly over the surface of the mushroom mixture. Drizzle half of the butter over the breadcrumbs.
Starting at the very bottom of the steak, carefully and tightly roll the steak, and secure the seam with toothpicks to hold it together.
Place the rolled steak in the center of a baking pan and brush the top of it with the remaining butter.
Bake until the probe inserted into the steak reaches an internal temperature of at least 145 degrees F, around 40 minutes. Your steak will be medium to medium well around this temperature. If you prefer it well done, continue cooking until it reaches an internal temperature of 155.
Remove the rolled steak from the oven and allow it to sit for 10 minutes. Cut the steak into 1-inch thick slices, remove toothpicks, and serve with easy roasted brussel sprouts if you desire.