Shuck the peas. In a large saucepan over medium heat, melt the butter. Add the onion and sauté until soft, 5 to 6 minutes. Try not to let it brown. Add the stock and salt, raise the heat, and bring the pot to a boil. Add the peas, cover, and return the pot to a boil. Lower the heat and simmer the peas until just cooked through, 2 to 5 minutes. depending on size. Transfer the peas and some of the liquid to a blender and purée until smooth. Add the remaining liquid and blend again. Pass through a coarse strainer or food mill. Serve warm, topping each bowl with 1 tsp. of the crème fraîche and 1/2 tsp. of the chopped mint.