Peel and cube 2 of the potatoes. Place them in a pot with just enough water to cover. Add 1 teaspoon salt and bring to a boil. Cook until fork-tender, about 10–12 minutes. Drain well, mash until smooth, and place in the refrigerator for 1–2 hours to cool completely.

When ready to make the boxty, peel and grate the remaining raw potato. Place the grated potato in a clean kitchen towel or paper towel and wring out as much moisture as possible—this step ensures your boxty will be crisp rather than soggy.

In a large mixing bowl, combine the chilled mashed potatoes, grated potato, flour, remaining 1 1/2 teaspoons salt, and baking soda. Pour in 1 1/2 cups of buttermilk and stir to combine. The mixture should be thick but scoopable; if it feels too dry, add more buttermilk, a splash at a time.

In a large skillet or frying pan, heat the vegetable oil over medium-high heat until shimmering.

Using a large spoon, drop portions of the potato mixture into the hot oil. Use the back of the spoon to flatten each portion into a pancake about 1/2 inch thick. Fry for 4–5 minutes on each side, or until golden brown and crisp. Adjust the heat as needed to avoid burning while ensuring the centers cook through.

Transfer the cooked boxty to a paper towel–lined plate to drain excess oil. Serve hot with a dollop of sour cream and a sprinkle of chopped green onions.