In a 5-quart Dutch oven or large pot over medium heat, heat the oil.
Add in the flour and cook, stirring constantly until the flour is the color of light caramels.
This should take about 5 minutes.
Add in onion, bell pepper, celery, garlic, and creole seasoning, stirring and cooking for another 5 minutes.
Add in chicken and sausage; cook for 3 minutes.
Gradually add in the chicken stock, whisking until the sauce is thickened.
Decrease heat to a simmer and cook, partially covered, for 30 minutes.
Refrigerate the filling for up to two days before baking; reheat to serving temperature before baking.
Make the crust: in a big bowl, whisk together the flour, baking powder, baking soda, and salt.
Cut in shortening, then stir in the buttermilk until the dough begins to form.
Turn the dough out onto a lightly floured board and roll it out ½ inch thick.
Cut out individual biscuits.
When ready to bake with the crust, transfer the filling to an ovenproof baking and serving dish; preheat oven to 425°; lay biscuits on top of the gumbo; brush biscuits with the melted butter and bake for 15-20 minutes, until golden.