In a sauce pan over medium heat, melt the butter.
Add the flour and whisk constantly for 1-2 minute (or until the mixture turns light brown).
Remove from heat. Gradually pour in the warm milk, whisking constantly.
Return the pan to the heat. Whisk another 3-5 minutes or until thickened. Again, remove from heat.
Add the Cajun seasoning and stir to combine.
Add the cheeses, stirring to melt.
Pour the sauce over the top of the cooked elbow macaroni. Stir to combine.
Add diced tomatoes and stir to combine.
Top with the sliced chicken breast.
Season with salt and pepper, and top with chopped parsley. Serve immediately.