Melt the butter in the skillet. Add the scallions and cook until softened, about 3 minutes. Add 1 1/2 cups of the chicken broth, the demiglace, bay leaf and 1/3 cup of the Marsala and simmer for 8 minutes. In a small bowl, whisk the remaining 1/4 cup of chicken broth into the flour until smooth. Stir the slurry into the sauce and simmer for 3 minutes. Discard the bay leaf.