In a small saucepan, whisk together the chicken broth, cornstarch and flour over medium heat until thickened, 1 minute. Whisk in the beer and bring to a simmer, whisking, 2 to 3 minutes. Whisk in the cheese; cover.
In a large skillet, heat the olive oil over medium-high heat. Add the hot dogs and cook, stirring, until browned, about 3 minutes; transfer to a plate. Add the cabbage and scallions to the pan, cover and cook, stirring occasionally, until wilted, about 5 minutes. Return the hot dogs to the pan.
Meanwhile, toast the bread and spread with the mustard. Stir 2 tablespoons cheese sauce into the hot dog-cabbage mixture and divide the mixture among the toasts. Top with the remaining cheese sauce.