In a large skillet over medium heat, add in unsalted butter. Once the butter is melted and starting to slightly bubble, sprinkle in the flour and whisk to combine. Cook for 1-2 minutes consistently whisking.
Once the roux is a light brown in color start whisking in the half and half slowly. As the roux absorbs the half and half it will begin to thicken.
Continue to pour and whisk increasing the amount of half and half being added until the roux is lump free.
Bring to a simmer and add in the white pepper, onion powder, nutmeg, salt and garlic.
Cook until the sauce coats the back of a spoon and is a thick smooth consistency. That should take 5-6 minutes over medium heat.
Once the sauce has reached the proper consistency, remove from the heat.
Whisk in 1 cup of the muenster cheese, parmesan cheese and the shredded mozzarella cheese.
Follow that by folding in the cooked shells and mix throughly.
Transfer the mixture to a rimmed baking dish.
Sprinkle the top with oregano and remaining 1 cup of muenster cheese. Drizzle with olive oil and bake at 350F for 30-45 minutes or until golden brown.
Allow to cool for 10-15 minutes and serve!