Shell the peas. Bring a large pot of water to a rolling boil over high heat. Prepare the icebath by filling a large bowl with ice water and partially submerging a colander in it. Boil the peas until they turn bright green. The sweetest, least starchy peas will only take a minute. Immediately drain the peas and submerge them in the ice bath in the colander. Once the peas have cooled, remove them from the ice bath and keep cool. Set the ice bath aside.

Bring 4 cups of water to a boil in an electric kettle, regular kettle or pot.

Dice the onions about ⅓-inch thick. Sautée in a large pot over medium-low heat in the butter until sweet and translucent but not beginning to brown, 7-10 minutes. Add salt and the peas and stir to coat them. Add the hot water and boil over high heat until the peas are tender, about 3-5 minutes. Keep tasting some of the larger peas to avoid cooking them any longer than necessary. Immediately pour the soup into a metal bowl and place it in the ice bath. Stir the soup frequently to cool it down quickly. Add more ice to the water if necessary.

Puree the soup in a blender until completely smooth. Depending on how powerful your blender is, you might have to strain the soup through a fine sieve. Add a cup of cold water to thin the soup, taste for salt and add as needed. Refridgerate.

Finely chop the mint, stir it into the yogurt and salt to taste.

Serve soup chilled with a dollop of the mint yogurt and some toasted (or better yet grilled) crusty bread.