Peel, slice and wash carrots and potatoes. Peel onion.
Put vegetables in a large casserole dish. Cover with water. Add bay leaves and salt. Bring to a boil and leave to cook for 30 to 40 minutes until tender when pierced with knife
Remove from heat. Reserve 250 ml. or 1 cup of cooking water. Drain and remove bay leaves.
Mash vegetables using an eletric potato masher.
Add butter and mix.
Add some cooking water until you get the texture you like most.