Preheat oven to 350°F. Position one rack in the middle of the oven, another below the broiler.
For the crostini, slice the baguette into 1/4" thick disks. Arrange in a single layer on a rimmed baking sheet and toast in the oven for about 10 minutes or until golden and crispy throughout.
While the crostini are toasting, drain artichoke hearts thoroughly and chop into small dice. If you have quarters that include some leaf pieces, remove any tougher leaves and dice the more tender leaves. Set aside.
Squeeze out as much water from the spinach as possible and set aside.
Add cream cheese, sour cream, Reggiano, 1/2 cup of the mozzarella, basil, salt, pepper, and garlic powder to a large bowl and mix thoroughly with your hands.
Stir artichoke and spinach into the mixture to combine, then add complete mixture to an 8" square baking dish and place on the center rack in the oven. Bake for about 20 minutes until hot and bubbly.
Turn on the broiler to high. Sprinkle the remaining 1/4 cup of the mozzarella over the top and broil for about 3-4 minutes or until the cheese turns golden in spots.
You can bring immediately to the party right in the baking dish. You can also make the recipe a day ahead, completing steps 1-6 and refrigerating the dip after it's cooled, covered in foil, saving the remaining mozzarella in an airtight container. Before your party, reheat the covered dip for 20 minutes at 350°F until hot, then remove the foil and proceed to step 7.