Bring a large pot of salted water to a boil. Add the 12 jumbo pasta shells and cook until al dente, about 9 to 11 minutes. Drain and set aside on a lightly oiled baking sheet to prevent sticking.
Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add chopped beef steak seasoned with salt, pepper, and 1 teaspoon Italian seasoning. Cook until browned and cooked through, about 5 to 7 minutes. Add minced garlic and chopped spinach, cooking until spinach wilts, about 2 minutes. Remove from heat and let cool slightly.
In a mixing bowl, combine 1 cup ricotta cheese, ½ cup grated Parmesan cheese, and 1 cup shredded mozzarella cheese. Add the steak and spinach mixture and stir until combined. Adjust seasoning with salt and pepper as needed.
Preheat the oven to 375°F (190°C). Using a spoon or small spatula, fill each cooked pasta shell with the steak, spinach, and cheese mixture. Arrange stuffed shells in a single layer in a baking dish.
Pour 1 cup Alfredo sauce evenly over the stuffed shells. Optionally, sprinkle extra mozzarella or Parmesan cheese on top. Cover with foil and bake for 20 minutes. Remove foil and bake an additional 10 minutes until sauce is bubbly and cheese is melted and slightly browned.
Remove from the oven and let rest for 5 minutes before serving. Serve with a fresh green salad or crusty bread.