Preheat oven to 375°F (190°C).
Slice the top third off the loaf of bread and hollow out the center, leaving a 1-inch border around the edges.
In a large bowl, mix together the spinach, sour cream, cream cheese, mozzarella, Parmesan, garlic, onion powder, salt, and pepper.
Add optional ingredients if using.
Fill the hollowed-out bread with the spinach dip mixture.
Place on a baking sheet and bake for 25 minutes, or until the dip is hot and bubbly and the bread is golden.
Let cool slightly, then slice and serve warm.