Bring a large pot of water to a boil on high, and add the pasta and salt. Cook according to the package’s directions until tender then drain the pasta. Turn off the stove
Rinse out the pot and then return it to the stove. On medium-low heat, melt the butter then add the onion. While stirring occasionally cook until softened and beginning to brown, about 5 minutes. Stir in the garlic and cook for 30 seconds.
Add the brisket to the pot, and cook until warm, about a minute or two. Pour in the heavy cream, then add the Cheddar and American cheeses. While stirring, cook until the cheese has melted, about 2-4 minutes.
Season the sauce with the pepper and cayenne, then return the cooked pasta to the pot. Stir until the pasta is well combined with the cheese sauce, then taste and adjust seasonings, adding more salt if needed.