Shell the peas into a bowl and set aside [you should have about 350g peas].
In a 3-litre saucepan, melt the butter over a low heat and sauté the onions for 10 minutes, tossing them back and forth until they gain a touch of colour. Take care that the butter does not burn; if it does, discard it, clean the pan and add some more fresh butter to continue the cookin