Peel potatoes, chop roughly. Cook in boiling salted water until tender, drain. Add butter, mash until smooth, add sour cream, milk, salt and pepper, beat until smooth.

Peel carrots, chop roughly, put in saucepan with crumbled stock cubes and peeled and chopped onion. Cover with water, bring to boil, reduce heat, simmer uncovered 15 to 20 minutes or until tender. Drain, reserve 2 tbsp of the carrot liquid. Put carrot mixture, reserved carrot liquid, salt, pepper, mustard and mayo into electric blender, blend until pureed, or mash until smooth. Put potato and carrot mixtures in layers in overproof dish. Dot extra butter over top of casserole, brush with extra milk. Bake in moderate oven 30 - 35 minutes or until top is golden.