Preheat oven to 325°F. Cut the crusts from the bread slices and discard. Cut the bread slices into small cubes. Toss with the melted butter and spread out onto a shallow baking pan. Bake for 17-22 minutes or until golden brown. Stir halfway through baking.
In a large soup pot, bring the chicken stock to a boil. Add the peas, parsley, mint, and scallions and return to a boil. Reduce the heat to medium and simmer for 5 minutes. While stirring the soup, gradually pour in the cornstarch-water mixture and bring back to boil, stirring occasionally, and then cook 2 to 3 minutes until the soup is smooth and thick. Using a hand-held or traditional blender, puree the soup until smooth. Strain through a sieve or food mill to remove the pea skins and then transfer back to the pot. Stir in the butter, one piece at a time, heating until the butter is melted. Season to taste with salt, black pepper, and nutmeg. Serve each portion of soup topped with a small handful of the buttered croutons.