Preheat the oven to 425 degrees.

Bring a large pot of water to a boil with 2 Tbsp of salt. Cook your macaroni according to package directions (but make sure it is el dente)—do not rinse under water after the pasta is cooked. Set aside.

Make the roux: In a medium saucepan, heat the butter on medium-low heat. After it has melted and begins to gently bubble, add the flour, and whisk consistently to incorporate. Smooth out any lump, and continue stirring frequently until the flour turns an off-white color and emits a nutty scent.

Stream the warmed milk into the pan slowly and continue to stir. When milk is incorporated and the sauce is smooth, add in the beer and continue stirring. Whisk gently over low heat until the sauce is smooth and thickened, 5-7 minutes.

When sauce is smooth, remove from heat and add in 1/3 cup cheddar cheese, Dubliner, mozzarella, pasta, bacon, pastrami, and 1/4 cup chives. Mix until all ingredients are incorporated, then transfer to baking dish or skillet. Sprinkle with remaining 1/3 cup of cheddar cheese.

Bake for 27-30 minutes, or until cheese bubbles and is slightly browned.

Remove from oven, sprinkle with remaining chives, and serve hot.