Prep everything beforehand. Dice the trinity (onion, celery, bell pepper), mince garlic, season shrimp with salt and pepper, etc.In a medium-heated heavy pot or dutch oven, fry sliced sausage with a little olive oil and set aside.In the same pot, add butter and about 1/4 cup of olive oil. When butter melts, quickly whisk in flour until fully incorporated. Using a wooden spatula, stir continuously (to prevent flour from scorching) until roux is brown, about 10 minutes.
Add trinity and garlic to the mix and cook for another 5 minutes or so. Make sure the roux doesn't stick to the bottom of the pan, so add more oil if you need to.Now, add the stock to deglaze the pot for a couple of minutes. Then, sprinkle in the Cajun seasoning mix and bring to a boil.
Let reduce uncovered, still in medium heat, for about 40 minutes or until liquid reaches a gravy-like consistency.In the end, stir in the sausage slices and raw shrimp for a couple more minutes. When it returns to boil, turn off heat and re-season accordingly.
Let simmer for another 5 minutes before serving.To serve, plate a bed of rice and smother with etouffee.