Peel and finely dice onions and garlic. Peel, wash and dice the potatoes. Heat the oil in a saucepan and sauté everything in it. Add 3/4 L water, stock and peas, bring to the boil. The peas should be well covered with water. Cover and cook for about 1.5 hours, stirring several times, if necessary, adding a little more water.
Wash and dry thyme and pluck except for something to garnish.
When the peas are done, add the butter and cream. Puree everything finely with a potato masher or with a hand mixer to taste. Leftovers can easily be reheated in the microwave.