Bring a pan of water to the boil and put in the two beetroots. Reduce to a simmer, put the lid on and cook for about 1 hour, or until a knife goes easily through.
Leave to cool for 10 minutes then peel and grate the beetroots into a bowl. Drain any excess moisture away and stir in the mustard.
On a serving dish, layer rye bread, lettuce, a spoonful of