Saute beef, onion, celery and garlic in a large pan until well done. Strain off fat. Boil potatoes until fork tender, about 20 minutes. Drain and mash with butter. Blanch carrots and add to beef mixture. Make a gravy with beef base and water. Whisk in enough flour to thicken, about 2 tbsp. When gravy comes to a boil, add beef and carrot mixture, reduce heat and simmer for 5 minutes. Spoon meat mixture into a 2 quart casserole. Spoon potatoes on top. Bake for 20 mins. at 350°F. Top with cheese and let sit until cheese melts from the radiated heat, about 5 mins.