Melt the butter and sauté the onion cubes until translucent. Add the peas, sauté briefly and then pour in the broth. Reduce the heat and cover and simmer over a medium heat for 10 minutes.
Wash and finely chop the mint. Add the cream to the peas, then finely puree everything in the broth. Season to taste with salt, pepper, nutmeg, lemon juice and mint.