Cook penne pasta according to directions on the box. Drain pasta and set aside.
Rinse chicken breasts and pat dry with paper towel, then place in a large resealable plastic bag. If the meat is thick, pound it down to about 1-inch thickness. Add the Cajun seasoning to the bag, seal bag, and shake thoroughly until chicken is evenly coated.
In a large skillet, melt 2 tablespoons of butter over medium heat. Add chicken to skillet, turning occasionally until cooked through.
In a separate skillet, melt 2 tablespoons butter over medium heat. Evenly sprinkle on the flour and whisk together. Keep whisking for another 5 minutes or so.
Slowly whisk in the milk, about 1 cup at a time. Add the cream cheese. Cook, whisking often, until it is uniform and reaches your desired consistency, about 5-10 minutes. (It will thicken a bit more when removed from heat.)
Stir in the lemon pepper, salt, black pepper and garlic powder. Pour cream sauce over cooked noodles or just add the pasta to the skillet with the sauce.
Slice chicken breasts into strips. Serve pasta on serving plates, top with chicken breast strips, and sprinkle on diced tomatoes, green onions, and Parmesan cheese.
Pasta amount in one box will vary by brand and variety.