Simmering the stew for longer will help the sauce reduce and thicken. The melty cheese should also give the sauce a thicker consistency. However, if you'd like to make it even thicker, you can make a cornstarch slurry by whisking together 2 teaspoons of cornstarch in 2 tablespoons of water. Stir in the slurry as the mixture simmers. Cook the recipe as usual.