1/2cupfreshly grated Parmesan cheese + 1/4 cup more for optional topping [See Step 7].
Preheat oven to 350 degrees F.
Prepare a rimmed baking sheet with aluminum foil, parchment paper, or a silicon liner (optional - for easy clean up) and set aside.
For the french bread, cut a large rectangle about 2 inches deep along the entire legnth of the loaf. Remove the rectangle you cut and, if necessary, continue to hollow out the loaf to create a bread bowl. Set loaf onto the prepared baking sheet and set aside.
Heat olive oil over medium-high heat. Add onions and sauté for until softened; about 5 minutes. Then reduce the heat to medium and add the bell pepper and garlic. Cook for another minute or two being careful not to burn the garlic.
Next, stir in the spinach leaves, tomatoes, paprika, salt, pepper, and mayonnaise until combined.
Finally, reduce the heat to low and add the cream cheese, sour cream, and all of the shredded cheeses ( minus the extra Parmesan for topping), and stir until fully melted.
Spoon the spinach dip into prepared French bread and top with reserved Parmesan cheese. Bake on a rimmed baking sheet for 25-30 minutes or until cheese is melted and the edges of the bread are just slightly golden brown.
Remove from the oven and allow the bread to cool about 5 minutes before slicing. This helps keep the topping in place and ensure your guests don't burn themselves on the hot cheese.
I use mini sweet bell peppers - a whole bag costs less than 1 bell pepper in my area. I use mult colors.
Sweet paprika or plain paprika may be substituted for the smoked paprika. For an extra kick, spicy paprika may be used as well.