Preheat the oven to 350°F (175°C).
Slice the sourdough loaf diagonally across without cutting through the bottom, then slice in the opposite diagonal to create a crosshatch pattern.
Mix artichoke hearts, spinach, sour cream, cream cheese, half of the mozzarella, Parmesan, garlic powder, salt, and pepper in a bowl until smooth.
Gently stuff the cheese and vegetable mixture into the cuts of the bread, pressing it deep into the crevices.
Sprinkle the reserved mozzarella cheese on top, allowing some to fall into the cuts.
Wrap the stuffed loaf in foil and place on a baking sheet. Bake for 10 minutes.
Remove the foil and bake for an additional 10 to 15 minutes until the cheese is melted and bubbly.
Allow the bread to cool slightly before serving warm and pulling apart to enjoy.