Get three shallow dishes. Fill the first bowl with flour. Beat an egg and 1 tablespoon of water into the second one. In the third dish, add seasoned breadcrumbs and parmesan cheese.
Season the chicken breasts with salt and black pepper. Dip the chicken pieces first in the flour then shake off the excess. Dip the chicken into the egg then shake off the excess. Finally, coat them in the breadcrumbs and shake off the excess.
In the meantime, bring a large pot of water to a boil. Once boiling, season with salt. Add the pasta and cook until al dente. Reserve at least ½ cup of pasta water and then drain the pasta in a colander.
In a skillet, heat oil over medium-high heat (hot but not smoking). Working in batches, cook the chicken breasts until golden brown and fully cooked, about 3 minutes on each side. Transfer them to a cooling rack.
Wipe off the extra oil from the skillet, leaving about a tablespoon of it. Melt the butter over medium heat and cook bell peppers, mushrooms, and onions for 3 minutes. Add the garlic and cook just until fragrant.
Add flour to the pan and stir for a minute.
Pour in chicken stock, heavy cream, cajun seasoning, cayenne pepper, and paprika. Bring the mixture to a simmer and let it thicken for about 5 minutes or until the desired consistency is achieved.
Lower the heat. Add grated parmesan cheese and stir to combine until smooth.
Add the cooked pasta, ¼ cup pasta water or more as needed and toss to combine. Taste and add more cajun seasoning, salt, and other seasoning if needed.
Serve pasta with sliced parmesan chicken, diced green onion, and parmesan cheese.