Soak the peas. Using a food scale for accuracy, place (225g) of dried marrowfat peas and 1 teaspoon of baking soda in a large, heatproof bowl. Add 3 cups (720ml) of boiling water. Stir the peas, lightly cover the bowl with a paper towel, and leave them to soak for at least 12 hours or overnight.
Wash the peas. After 12 hours, drain the peas with a colander and rinse thoroughly under cold water to remove the baking soda.
Simmer the peas. Transfer the peas to a saucepan and add 2 ½ cups (600ml) of water. Bring to a boil, then reduce the heat to medium. Simmer uncovered for approximately 25-30 minutes, stirring occasionally, until the peas are tender.
Remove the skins. Periodically skim off and discard any loose skins that float to the surface. While this is a matter of personal preference and most skins will eventually break down, removing the larger ones ensures a smoother, more refined texture.
Add the seasonings. Once the mixture has thickened to your liking, remove the pan from the heat. Stir in the salt, black pepper, softened butter, and fresh mint.
Add the liquids. Once the butter has melted, fold in the lemon juice and heavy cream. Let the peas rest for 15 minutes. During this time, they will continue to thicken, and the flavors will meld. While they rest, boil ½ cup of water in the microwave.
Adjust the texture. Before serving, gradually stir in the hot water (¼ cup at a time) until you reach your preferred consistency. For a traditional texture, you will likely use the full ½ cup.