Dice the onion, mince the garlic, and slice the sausage into thin coins. Remove the stems and seeds from the bell peppers, then thinly slice.
Heat a large cast-iron pan or pot over medium-high. Drizzle in 1 tablespoon oil. Add the sausage coins and sear, stirring occasionally, until browned. Transfer to a bowl with a slotted spoon, leaving the oil behind. Set aside.
Add 1 tablespoon olive oil to the pan and stir in the diced onion for 1 minute. Add sliced peppers and mushrooms, and sauté until crisp on the outside, about 6–8 minutes. Add 1 tablespoon garlic and sauté for 1 minute. Transfer vegetables to the bowl with sausage.
Cook pasta according to package directions, subtracting 1 minute. Generously salt the boiling water with 1 teaspoon salt per 4 cups water.
In the same pan, reduce heat to medium and add butter. Once melted, sprinkle flour and whisk until a roux forms. Cook, whisking constantly, for 1 minute. Gradually pour in 1/2 the chicken stock. Whisk until thick. Gradually add remaining stock and milk, 1/2 cup at a time, whisking constantly.
Increase heat to high and bring the sauce to a boil, stirring frequently. Reduce to a gentle simmer until the sauce thickens, about 5–8 minutes. It should be thick enough to draw a path across the back of a spoon that stays. Melt the cream cheese in the microwave until very soft. Add to the sauce along with the Cajun seasoning, oregano, paprika, salt, and pepper. Whisk briskly until combined.
Remove the cream sauce from heat. Stir in the cooked pasta, sausage, and vegetables, with any accumulated liquid. Toss until well coated. Gradually add Parmesan cheese, a few tablespoons at a time, until integrated. Taste and adjust seasoning. Garnish with parsley, if desired, and serve immediately. Optionally, grate more cheese atop individual bowls with a zester.
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Notes
Note 1: Any smoked skinless sausage works great. I usually use hardwood smoked turkey sausage to keep it lighter.Note 2: Most penne packages are either 12 ounces or 16 ounces. If you have a 16-ounce package, remove 1 cup, which is 4 ounces. This will leave you with 12 ounces, or 3 1/2 cups, which is the perfect amount.Note 3: Packaged grated Parmesan from a can is very salty and won't melt well. Grate a block of Parmesan on a zester, or use finely grated Parmesan.Storage: For optimal flavor, enjoy this dish immediately. Store leftovers in an airtight container in the fridge for up to 3 days. When reheating, add a splash of milk or chicken broth to loosen the sauce, as the pasta will absorb some moisture while stored. Freezing is not recommended due to the dairy content.