Mix all ingredients together well. Refrigerate.

Chop half of the potatoes into large dice, place in a medium saucepan, salt generously, and cover with cold water by 1 inch. Bring to a boil over high heat, reduce heat to low, and simmer potatoes uncovered until fork tender, about 8 minutes. Drain, return potatoes to the pot, add 1/4 cup of the milk, and mash until the potatoes are smooth; set aside.

Meanwhile, grate the remaining potatoes on the large holes of a box grater. Toss with 1/4 teaspoon of the salt and place in a fine mesh strainer over a medium bowl until the mashed potatoes are ready.

With a plastic spatula, press the grated potatoes against the sides and bottom of the strainer to remove any liquid. Add the grated potatoes to the mashed potatoes (no need to stir though).

Place egg, remaining 1/2 cup milk, flour, pepper, and remaining 1 teaspoon salt in a large bowl and whisk until smooth, about 10 seconds. Add potatoes and stir until evenly incorporated.

Heat a large nonstick frying pan or griddle over medium heat.

Once the pan is ready, add enough butter to lightly coat the bottom when melted. Drop 3 dollops (about 1/4 cup each) of the batter into the pan and spread each to about 1/4 inch thick. Cook until the pancake bottoms are golden brown, about 4 to 5 minutes. Flip and cook the other side until golden brown, about 4 to 5 minutes more.

Place on a baking sheet and set in the oven to keep warm.

Repeat with remaining butter and batter.

Top each boxty with a dollop of salmon spread and garnish with extra chopped dill and some sliced green onions.