Make the Cajun-style slow roasted onions. When done, let cool, then use a paper towel to remove the excess olive oil/butter mixture.
Chop the roasted onions, peppers and celery into small pieces.
Preheat oven to 350 degrees F.
Heat an 8-inch enameled cast iron skillet over medium heat on the stove.
Melt the butter in the skillet, then add the tapioca starch. Keep stirring for a minute or so until a very light blond roux is achieved.
Add the milk and cheeses, and stir until well-combined.
Pour the chopped roasted onions, parsley flakes, onion powder, garlic powder, paprika, hot sauce, and SP&C to taste into the mixture.
Stir until very well-incorporated.
Remove from heat for about 5 minutes. Since the mixture is already so hot, putting it straight into the oven will cause it to bubble and burn along the top in some places very quickly. Removing it from the heat for a bit lets it cool down a bit, which helps the entire baked Cajun French onion dip heat evenly in the oven.
Bake for 15 to 20 minutes, or until browned and bubbly.