In a medium saucepan over high heat, blanch the shucked peas in boiling salted water until their color brightens, or 20-30 seconds. Drain the peas and immediately plunge them into a bowl of iced water. After the peas have chilled, drain and set aside.
In a medium saucepan over medium-high heat, combine the reserved pea pods, onion, garlic, potato and stock. Boil until the stock is reduced to three-quarters of its original volume and the potato is very tender or about 40 minutes.
Transfer to a blender or food processor. Add half of the reserved peas. Blend until velvety smooth, adding water if the soup is too thick to easily purée. Pass through a fine strainer. Add the ice cubes and creme fraiche and stir until the ice cubes have melted. Season with salt and lemon juice and place in the fridge to fully chill, about 1 hour. To serve the soup warm, return it to the saucepan and warm fully over low heat, for about 10-15 minutes.
Divide the soup into four soup bowls. Top each with the mint leaves, pea flowers and shoots and lemon zest. Serve immediately.