In a large pot, bring salted water to a boil. Add the fettuccine and cook according to package instructions until al dente. Drain and set aside, reserving 1 cup of the pasta water.
In a bowl, toss the sliced chicken with Cajun seasoning until evenly coated.
In a large skillet, heat the olive oil over medium-high heat. Add the seasoned chicken strips and cook for about 6-7 minutes, or until golden brown and cooked through. Remove the chicken from the skillet and set aside.
In the same skillet, add the chopped onion and cook for 2-3 minutes until translucent. Then, add the minced garlic and cook for another 1 minute until fragrant.
Pour in the chicken broth and bring to a gentle simmer. Stir in the heavy cream and let it simmer for about 3-4 minutes, allowing it to thicken slightly.
Add the cherry tomatoes and spinach to the sauce. Stir until the spinach wilts and the tomatoes soften. Return the chicken to the skillet and mix well.
Add the drained pasta to the skillet, tossing to combine. If the mixture is too thick, add reserved pasta water a little at a time until you reach your desired consistency. Season with salt and pepper to taste.
Plate the creamy Cajun chicken pasta, garnishing with chopped parsley and a sprinkle of grated Parmesan cheese if desired.