Peel the potatoes, cut into small cubes and simmer in vegetable stock for 8-10 minutes. Add the peas, bring to the boil and simmer for 3 minutes. Strain everything and collect the liquid in a cup.
Put the potatoes with the peas back in the pot, stir in the butter and just roughly mash everything with the masher (DO NOT puree!) If the mixture is too thick, stir in a little cooking liquid. Season to taste with lemon juice and pepper, salt is usually unnecessary by cooking with vegetable stock.