Place potatoes in a large pot and cover with cold water. Bring to a boil and add 2 tablespoons salt. Reduce heat to maintain a simmer and cook until fork-tender, 13 to 15 minutes. Drain. Transfer to a large bowl and mash with a potato masher.Advertisement
Meanwhile, place dulse in a medium bowl and cover with cold water. Let soak until soft and pliable, about 15 minutes. Rinse and finely chop.
Melt butter in the pot over medium-high heat. Add the dulse and cabbage. Cook, stirring occasionally, until the cabbage is wilted, 2 to 5 minutes. Add scallions and cook, stirring constantly, for 1 minute more. Reduce heat to low. Add the mashed potatoes, milk and the remaining 1/2 teaspoon salt. Stir until well combined.
2 starch, 1 1/2 fat