Bring the peas to the boil in 600 ml of water. Cook until soft for about 1 hour over a mild heat.
In the meantime, peel the onions, cut or slice into 5 mm thick rings and mix carefully with the oil. Spread out on a greased baking sheet and fry in the preheated oven at 220 ° C on the 2nd rack from the bottom for about 20 minutes - turn once after 10 minutes.
Puree the peas with the cooking water. Boil down to a puree over a low heat while stirring for about 10-15 minutes. Stir in the butter and season the puree with salt, pepper and nutmeg. Arrange the puree with the onion rings and serve.