Bring a pot of salted water to a boil. Cook penne according to package directions.
Melt butter over medium heat. Add flour; stir for 1 minute. Add half-and-half; stir until thickened, 6 minutes. Remove from heat; add Parmesan, salt and pepper. Cover.
Place chicken between 2 sheets of plastic wrap; pound with a rolling pin to 1/2-inch thickness. Rub Cajun spice and cayenne, if using, evenly over chicken. Warm oil in a nonstick skillet over medium-high heat. Add chicken and cook, turning once, until no longer pink in center, 6 to 8 minutes. Slice chicken.
In a bowl, toss pasta with sauce. Top with chicken and tomatoes and serve.