Preheat oven to 350 degrees. Slice top from the head of garlic to expose cloves. Place in a small ovenproof dish, drizzle with olive oil, cover tightly with foil and bake for 30 minutes. While garlic roasts, make the remaining ingredients.
Turn oven to 400 degrees. Set water to boil in a large stock pot.
In a medium skillet, cook the pancetta over medium-high heat, stirring often, until crispy and browned on the edges. Set aside on a plate lined with paper towels.
To make the brioche crust, melt the half of the butter in the microwave or on low heat on the stove, then mix with the mustard and cayenne pepper in a medium bowl. Toss with the brioche bits and set aside.
When the water in the stock pot is at a rolling boil, add the pasta and cook according to package directions. Drain.
While pasta is cooking, make the sauce: In a medium saucepan on low-medium heat, melt the remaining butter. Incorporate the flour, using a whisk. Continue to whisk for about three minutes, then whisk in the heavy cream and milk, a little at a time. Bring to a simmer, continuously whisking. Add the cheese gradually, stirring constantly until all cheese is melted and the sauce is smooth. Squeeze roasted garlic cloves from their skins and mash with a fork. Add mashed garlic to the cheese sauce along with salt and pepper to taste.
Assemble and Bake: Mix the pasta and sauce, making sure all noodles are coated. Fold in the pancetta and arugula. Pour into a large casserole dish. Sprinkle with brioche crumb topping, and bake until the topping turns golden brown, about 15-20 minutes. Serve.