9ouncesdried marrowfat peas(there is no substitute, marrowfat peas are larger, have a higher starch content, and contribute the right flavor and texture for traditional mushy peas)
Dissolve the baking soda in boiling water. Place the dried marrowfat peas in a bowl and pour the boiling water over them so they're covered by at least 3 inches of water. Give the peas a stir then leave them to soak for at least 12 hours.
Drain and rinse the peas and place them in a pot with about 3 cups of water. Bring to a boil, cover, reduce the heat to medium-low, and simmer for about 30 minutes, stirring occasionally, or until the desired consistency is reached. Once the peas have fully broken down add the salt. Note: How long you have to simmer the peas will depend on the particular crop and the age of the peas.
If the peas are too watery, continue to simmer with the lid off until it thickens to your liking. If the peas are too thick, add a little water. Taste again and add more salt if needed. Once the peas are done they will start to thicken the longer they sit. If reheating them later or the next day, add a little more water.