Heat sauté pan over medium heat. Add a touch of cooking fat and sear sliced sausage on both sides. Remove and set aside.
Add remaining fat, lower heat, whisk in flour to make a roux. Cook, stirring constantly, until dark brown.
Stir in diced celery, bell pepper, onion (half), and sliced garlic (half). Cook for 2–3 minutes.
Add Cajun seasoning + truffle salt mix, black pepper, and tomato purée with thyme and bay leaves. Cook for 1–2 minutes.
Return sausage to the pan and stir well.
Pour in shrimp stock, cover, and bring to boil.
Reduce heat to medium-low, uncover, and simmer 30 minutes, stirring occasionally.
In the final 5 minutes, add shrimp and cook until just opaque.
Heat saucepan with cooking fat over medium heat.
Add reserved onions & garlic, cook until translucent.
Stir in rinsed rice, toast for 2 minutes. Mix in butter until melted.
Add tomatillo purée, cook until evaporated.
Pour in chicken broth, parsley sprigs, and pinch of salt. Cover.
Bring to boil, reduce to low, and cook until liquid nearly absorbed.
Turn off heat, keep covered 10–15 minutes to finish steaming.