Preheat the oven to 350-degrees F, with rack in the middle position.
Meanwhile, cook the macaroni according to the package directions. Rinse with cold water and set aside.
In a large skillet, melt the butter over medium-high heat, add the onions and garlic, stirring occasionally until lightly browned. Add the ground beef, breaking it up with a wooden spoon or spatula.
Lower the heat to low, stir in the fish sauce, Accent (if using), salt and cook for about 2 minutes. Raise the heat to medium and cook, stirring occasionally to incorporate the juices, until the beef is cooked through, about 5 minutes more.
Off the heat, stir in the reserved cooked macaroni, then transfer the mixture to a 9x13 pan. Scatter the cheese evenly across the top. Cover the pan tightly with aluminum foil, leaving some space between the top of the macaroni and the foil. Bake until the cheese is melted, 10-15 minutes.