Sauté the shrimp shells in a pot with a splash of oil for about 3-5 minutes to release their flavor.
Add aromatics like garlic and saute for an additional 2 minutes.
Top it off with 10 cups of water to create a savory stock by simmering for 30 minutes.
Strain and set aside.
In a large pot over medium heat, melt butter and olive oil together.
Add the chopped onion, green bell pepper, and celery.
Sauté while stirring occasionally until the vegetables are softened and the onions become translucent, about 5 minutes.
Sprinkle in the flour, stirring continually, until you achieve a golden-brown roux.
This should take about 3 minutes.
Add the shrimp stock slowly while stirring, ensuring a smooth consistency.
Season with cayenne pepper, paprika, black pepper, and salt to taste.
Let it simmer for 10 minutes, stirring frequently, until the sauce thickens.
Add in the shrimp and let it simmer gently until they just turn opaque.
Garnish with parsley and green onions just before serving.
Serve hot over a bed of cooked white rice.