In a large Dutch oven or pot, melt butter over medium heat. Add potatoes, carrot & cabbage & cook, stirring, for about 5 minutes. Add flour & cook, stirring, for about 1 minute more. Add chicken broth, milk, mustard, thyme, salt & pepper. Stir to combine. Bring mixture to a boil, then reduce heat to simmer. Simmer 15-20 minutes, or until vegetables are tender. Stir in corned beef & cheese, cook, stirring until cheese has melted. Taste to adjust seasoning as necessary. Remove from heat & allow to cool 15-20 minutes.