Melt butter in 2.5-quart saucepan over medium heat. Add onions and cook and stir until onion is soft, about 3 minutes. Add flour and cook and stir for 3 minutes on low heat. Do not brown. Slowly stir in chicken stock. Add mustard and carrot pulp and hot milk. Bring to a boil, then reduce heat and simmer 5 minutes. Remove from heat and add sliced cheese. Stir until cheese is melted.