Grate or cut the metton into small pieces so it melts more easily.
Pour the water (or the water-and-milk mixture) into a heavy-bottomed saucepan, then add the metton.
Heat over low heat, stirring constantly with a wooden spoon.
As soon as the metton begins to melt, add the butter in pieces.
Continue stirring until smooth and uniform. The temperature should reach 80-90 °C.
Season with salt to taste. Taste as it melts — the metton may already be salty.
Remove from the heat once the texture is runny and uniform.
Serve hot over potatoes with Morteau sausage, or let it cool and spread it on bread.
Notes
Without melting salts, the texture may be slightly less smooth than in the commercial version, but the flavor will be better.For a garlic version, add 2 minced garlic cloves halfway through cooking.Cancoillotte keeps for 5-7 days in the fridge. Reheat gently to restore its smooth, runny texture.For an even smoother texture, add 1 g of baking soda along with the water.