Boil pasta: In a large pot, bring chicken stock and 1 Tbsp olive oil to a rolling boil. Add penne and cook to al dente (package time minus 1 minute). Drain (reserve 1/2 cup starchy stock if desired).

Brown sausage: Heat 1 Tbsp olive oil in a large skillet over medium–high. Add sliced sausage; cook 5–7 minutes until browned. Transfer to a bowl.

Cook shrimp: Toss shrimp with 1 Tbsp Cajun seasoning, a pinch of salt and pepper. Sauté 2–3 minutes per side until just pink. Transfer to the bowl with sausage.

Start sauce: Reduce heat to medium; add butter. When melted, pour in half-and-half (or cream). Bring to a gentle simmer, then reduce to low and cook 5 minutes, stirring, until slightly thickened.

Finish sauce & combine: Stir in cheeses and remaining 1 Tbsp Cajun seasoning until melted and smooth. Add cooked penne, sausage, and shrimp; toss on low heat 1–2 minutes until glossy and coated. If too thick, loosen with a splash of reserved starchy stock.

Taste & serve: Adjust salt/pepper/Cajun to taste. Garnish with parsley (and red pepper flakes if you like heat). Serve hot.