Wash the beets and wrap them in aluminum foil. Bake in a preheated oven at 428°F (220 °C) for approximately 40 minutes, or until ready.
Allow to cool for 10 minutes, peel and cut into slices. In the meantime, clean the potatoes and cut them into slices. Boil the sliced potatoes and drain them of their water. Boil the peas separately.
Put the potatoes, beets, peas and finely chopped onions in a salad bowl. Put the egg yolk, mustard and vinegar in a blender, or beat with an immersion blender. While beating, pour in a thin trickle of olive oil. Transfer to a small bowl and add the cream.