Chop the bacon and the leeks. Sauté them in the butter in a large saucepan for about 8 minutes. Remove a couple of teaspoons, and set aside to use as a garnish when serving. Add the peas, mint if using, and the stock. Bring just to a boil, add some salt and pepper, and reduce heat to low and simmer for 10 to 15 minutes.
Purée the mixture in a food processor or blender, then pour back into the saucepan and reheat.
When reheated, remove from the heat, stir in the yoghurt or cream or milk and serve. Garnish each bowl with a dollop of yoghurt and a sprinkle of the bacon and leek you set aside earlier.
The peas can also be fresh. Instead of yoghurt, you can use cream, milk, or crème fraîche. For the mint, use a handful of chopped mint leaves, or a ½ teaspoon of dried mint. Or, omit mint altogether.