Preheat oven to 400°F (200°C).
If the cream cheese is cold, let it sit at room temperature 15–20 minutes or microwave in 10–15 second intervals until soft enough to mix easily.
Thaw the frozen spinach and squeeze out as much liquid as possible using your hands or a clean kitchen towel.
Drain the 14 oz can of artichokes and chop any large pieces so they are bite-sized.
In a medium mixing bowl, beat together 6 oz softened cream cheese, 5 Tablespoons sour cream, and 5 Tablespoons mayo until smooth.
Add the drained, chopped artichokes, squeezed spinach, 1 pkg Ranch dressing seasoning mix, 1/4 teaspoon salt, 1/4 teaspoon black pepper, and 1/4 cup of the 3/4 cup shredded mozzarella. Stir until well combined. Reserve the remaining 1/2 cup shredded mozzarella for topping.
Slice the 1 Italian loaf bread lengthwise (into two long halves) and place the cut sides up on a baking sheet.
Brush the cut sides of the bread with 1/4 cup melted butter.
Cover the bread loosely with aluminum foil and bake in the preheated oven for 7 minutes to warm the bread.
Remove the foil, spread the spinach-artichoke mixture evenly over the cut sides of the bread, and sprinkle the reserved 1/2 cup shredded mozzarella over the filling.
Return the baking sheet to the oven and bake 12 minutes, or until the cheese is melted and the topping is hot and bubbly.
Remove from the oven and let the bread cool to room temperature before slicing and serving.