Sauce -- In a medium pot on medium heat, add the butter and melt, then add the flour and mix well to make a roux. Slowly add in the milk and bring to a low boil to let the sauce thicken. Then add in the cheeses, salt, pepper and cajun seasoning to taste. Cook just until melted.
Pancetta -- In a medium size saute pan, add 1 tablespoon of the butter and saute the pancetta. NOTE: If using bacon, skip the butter, the bacon has enough fat. Saute until golden brown.
If you are using the bacon, reserve 1 teaspoon of the bacon drippings and omit the butter when sauteing the shrimp and vegetables.
Vegetables and Shrimp -- Now, to that same pan add the vegetables and cook until tender, 3-4 minutes. Then add the shrimp and stir just to mix everything together. Remove immediately, the shrimp will continue cooking in the oven.
Pasta -- Cook the pasta according to directions - al dente, drain well, return to the pot.
Combine -- In the pot with the pasta, add the shrimp and vegetable mixture, the cheese sauce, bacon and mix well. Pour into a 13x9 casserole dish lightly sprayed with pam or a non-stick spray.
Topping -- Melt the butter in a measuring cup in the microwave and add the cracker crumbs and parsley and mix well. Top over the casserole dish.
Bake -- In a preheated 350 degree oven, bake the dish covered for 20 minutes, uncover and cook another 10-15 until golden brown and bubbly. Remember everything is already cooked, it doesn't take too long. Enjoy!