Bring a large pot of salted water to a boil. Add the linguine and cook according to the package directions until al dente. Before draining, reserve about ½ cup of the pasta water. Drain the pasta and set it aside.

While the pasta cooks, heat 1 tablespoon of butter in a large skillet over medium-high heat. Add the chicken breast and season lightly with salt, black pepper, and Cajun seasoning if using. Cook for 5 to 7 minutes, stirring occasionally, until the chicken is golden brown and cooked through.

Remove the chicken from the skillet and set it aside. Add the sliced Cajun chicken sausage to the same skillet. Cook for 4 to 5 minutes, turning occasionally, until the sausage is browned and slightly crispy around the edges. Remove the sausage and set it aside with the chicken.

Reduce the heat to medium and add the remaining 2 tablespoons of butter to the skillet. Once melted, add the minced garlic and cook for about 1 minute until fragrant. Sprinkle in the flour and whisk constantly for 1 to 2 minutes to cook out the raw flour taste.

Slowly whisk in the milk and heavy cream. Keep whisking as you pour so the sauce stays smooth. Bring the sauce to a gentle simmer and cook for 3 to 4 minutes, or until it begins to thicken.

Lower the heat to medium-low and stir in the Parmesan cheese until melted and creamy. Taste the sauce and adjust with salt, black pepper, and additional Cajun seasoning if needed.

Add the cooked chicken, browned sausage, and linguine back into the skillet. Toss until the pasta is fully coated in the Alfredo sauce. If the sauce is too thick, add reserved pasta water a tablespoon at a time until it reaches your desired consistency.

Garnish with chopped fresh parsley and serve immediately while the pasta is warm and creamy.