Makes 8 servings

To make the croutons:

1. Preheat the oven to 350°F.
2. Combine the cubed bread, melted butter, and salt in a mixing bowl and toss them until the bread is fully coated.
3. Place the cubes in a single layer on an unlined baking sheet and toast them in the preheated oven for 10 minutes. Remove them from the oven, let them cool, and store them in an airtight container. The croutons can be made up to three days in advance.

To make the soup:

1. In a large, heavy stockpot over medium-high heat, melt the butter. When the foaming has subsided, add the leeks and sauté them until they are soft, 12 to 15 minutes. Add the stock and cream, bring the mixture to a boil, and then turn it down to a simmer. Add the peas and lettuce, and simmer the soup until the peas are just tender, 8 to 10 minutes.
2. Remove the soup from the heat, and puree it in batches, if necessary, in the bowl of a food processor fitted with the metal blade. Add the mint, salt, and pepper to the soup, and serve it immediately, or store it in the refrigerator for up to three days before reheating and serving. Scatter the croutons on top just before serving.