Mince your onion and garlic. Melt the butter in a heavy-bottomed saucepan and add your onion cook until the onion is soft — about 7 minutes, stirring constantly then add your garlic and cook until fragrant. Mix in your brisken cooking juice, Stir in the flour and cook, stirring constantly, until it forms a paste and bubbles a bit, but don't let it brown — about 2 minutes.
Add your milk Little by little and you have a thick sauce.

Boil water and cook the pasta.

Add your seasonings and Bay leaves and let the sauce simmer for 5 more minutes, cut the heat remove the bay leaves and add the Cheese stir until cheese is melted.
Add your Leftover brisket and mix gently and your cooked pasta, serve and enjoy.