Heat the oven to 350°F. Slice the top from the bread boule, hollow out the center while leaving about a 1-inch-thick shell, and cut the removed bread into cubes.

Thaw the frozen spinach and squeeze or press it very dry. Drain and finely chop the artichoke hearts, removing as much excess liquid as possible.

Mix the softened cream cheese, sour cream, mayonnaise, garlic, salt, pepper, and optional red-pepper flakes or onion powder until smooth.

Stir in the spinach, artichokes, Parmesan, and 3/4 cup of the mozzarella. Spoon the mixture into the prepared bread bowl and sprinkle the remaining 1/4 cup mozzarella over the top.

Place the filled bread bowl and bread cubes on a baking sheet. Bake the dip for 20–25 minutes, until hot and bubbling and the cheese is melted.

Toast the reserved bread cubes on the same baking sheet for about 8–10 minutes, turning once, until lightly crisp. Loosely cover the boule with foil if the crust begins browning too quickly.

Remove the bread bowl from the oven and let it rest for a few minutes so the dip settles slightly. Serve immediately with toasted bread cubes, crackers, and fresh vegetables.