Preheat oven to 325 degrees. Generously spray the inside of your lasagna-sized dish with vegetable oil.
While making the cheese sauce, boil salted water for your elbow macaroni. During the sauce-making, cook your pasta al dente (just done, a little bit firm), drain, and put back in the pot.
In a thick-bottomed sauce pan, melt butter until bubbly over medium heat. Add the flour and mix briskly with a wooden spoon until it forms a kind of dough until smooth.
Add very small amounts of the milk several times, mixing the "dough" after each addition until it gets smooth again. Keep adding the milk until all of it is used. (If you have lumps after all the milk has been added -- very common -- use a hand blender to smooth the mixture.) Keep stirring the sauce over medium heat until it thickens, about 5-10 mins.
Add the shredded cheddar & Gouda (or Gruyere) cheeses, reserving ¼ cup of the cheddar cheese for assembly. Stir until completely melted. Add more milk if the sauce is too, too thick. (You want it thick, but not so thick you can barely stir it.)
After the cheese melts, taste the sauce and add salt & pepper to taste. You can also add more cheese at this step if you want a cheesier taste.
After your macaroni has been drained and put back in the pan you cooked it in, pour in the cheese sauce and mix until all the macaroni is well-coated. Test taste again and see if you need more salt at this point, in case your macaroni was under-salted.
Pour HALF the mac and cheese mixture into the sprayed casserole dish and spread out evenly. Sprinkle with a layer of the reserved shredded cheddar. Then pour the remaining half on top. (If you are making the large batch, you should have enough to make an extra small casserole dish to cook later or freeze.
While the mac & cheese mixture is still hot, layer sliced Swiss cheese to completely cover the entire casserole. Sprinkle the Panko and plain bread crumbs evenly on top to coat, then spray the bread crumbs with a light coating of the vegetable oil.
Let the dish sit for five minutes (or overnight in the fridge if preparing in advance). Bake at 325 degrees for 25-30 minutes until bubbly and the cheese is slightly browned. Allow more time for cooking if you refrigerate overnight before cooking, or let the dish come to room temperature -- about an hour -- before baking.