Peel and cube 2 potatoes. Cube the remainder of the potatoes and place in a pot with just enough water to cover them. Add 1 teaspoon salt. Boil until fork tender. Drain and mash potatoes and place in refrigerator for 1-2 hours.
When ready to make peel and grate the rest of the potatoes. Place in a paper towels and wring to remove excess moisture. Combine mashed potatoes, grated potatoes, flour, salt, baking soda, and 1 1/2 cups buttermilk in a large bowl. If mixture is too dry add a splash more buttermilk.
Heat oil in a large frying pan over medium-high heat. Place large spoonfuls of the potato mixture into the oil, pressing down to flatten each one. Fry for 4-5 minutes on each side, or until the boxties are getting browned. Place on paper towel to drain.
Serve with a dollop of sour cream and some green onions scattered over the top.