Halve the chicken breasts horizontally (resulting in two thin fillets) then season generously on both sides with the Cajun seasoning.
Heat a large frying pan over medium-high heat. Add a splash of oil then add the chicken. Cook for a few minutes per side until golden brown and cooked through. Remove and set aside.
By this time, the water should be boiling. Add the pasta and cook until al dente. Reserve 1 cup of cooking water before draining.
In the same pan the chicken was cooked in, add the butter, onion and garlic. Cook for a few minutes until soft and translucent.
Add the Cajun seasoning and stir into the onions. Pour in the cream and lemon juice then bring to a gentle simmer. Cook for a few minutes until the sauce thickens enough to coat the back of a spoon. Reduce the heat and add the Parmesan. Stir until the sauce is smooth then season to taste.
Toss the drained pasta with the creamy sauce. Add a few tablespoons of pasta water. Stir to coat the pasta in the sauce, and add more water if you want it to be saucier. Feel free to add more grated Parmesan too. Slice the chicken thinly then add to the pasta.
Serve topped with chopped parsley and extra Parmesan.