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.
While the pasta is cooking, cut the chicken breasts into bite-sized pieces. Season with Cajun seasoning, salt, and pepper.
In a large skillet, heat olive oil over medium heat. Add the seasoned chicken and cook for about 5-7 minutes, or until the chicken is cooked through and no longer pink in the center. Remove chicken from the skillet and set aside.
In the same skillet, add the chopped onion and sauté for 3-4 minutes until translucent. Then add minced garlic and cook for another minute until fragrant.
Reduce the heat to low, then add the heavy cream to the skillet and stir to combine. Slowly add the grated Parmesan cheese, stirring until melted and smooth.
Add the cooked chicken and cherry tomatoes into the sauce, stirring until everything is well coated. Then, add the cooked fettuccine and toss to combine.
Taste and adjust seasoning with additional salt and pepper if needed.
Divide the creamy Cajun chicken pasta among plates and garnish with fresh parsley if desired.