Season the chicken breasts with salt and pepper. Heat a large deep sauté pan or skillet over medium-high heat and add enough high heat cooking oil to coat the bottom of the pan.
Sear the chicken breasts in batches for about 2 minutes on each side, or until browned and cooked through. Remove the chicken from the pan and set it aside.
Reduce the heat to medium and add the butter to the same pan. Once melted and bubbling, add the shallot, garlic, and tomato. Season lightly with salt and sauté until the vegetables begin to soften and release fragrance.
Add the Cajun seasoning and sliced mushrooms to the pan and sauté for about 30 seconds, until fragrant.
Stir in the rigatoni pasta and pour in the water and stock. Increase the heat to medium-high and bring it to a boil. Reduce the heat to medium, cover the pan with a lid, and let it cook for 5 minutes.
Remove the lid and add the heavy cream to the pan. Bring it to a boil over medium-high heat, then reduce the heat to low and let it simmer for 5-8 minutes, or until the sauce has reduced and the pasta is cooked al dente. Stir occasionally to prevent sticking.
While the pasta is simmering, cut the cooked chicken into cubes. Add the chicken cubes and lemon juice to the pan, stirring them into the sauce.
Finally, stir in the grated Parmesan cheese and continue heating the sauce over medium-low heat until the cheese melts and emulsifies into the sauce.
Remove the pan from heat and serve the creamy Cajun chicken pasta hot. Sprinkle with additional grated Parmesan cheese and freshly chopped parsley for garnish.