Prepare the Boxty (make up to 1 day in advance):

1. Wrap up the grated potatoes with a clean kitchen towel, and squeeze tightly to remove as much liquid as possible.
2. In a large bowl, mix together the grated potatoes, mashed potatoes, flour, green onions, baking powder, and salt. Stir in the buttermilk until you have a smooth, thick mixture.
3. In a large heavy-bottomed skillet set over medium-high heat, melt 1 tbsp butter. Working In batches of 3-4 cakes at a time (depending on the size of your pan), spoon the batter into the pan using a heaping 1/4 cup measure for each boxty. Using a spatula, flatten each cake to roughly 1/2-inch thick. Cook for 4-5 minutes per side, turning just once, until the cakes are crispy and golden-brown. Transfer the finished cakes to a baking sheet in a 300F oven to keep warm while you work on the remaining elements.
4. (If making in advance, let the cakes cool completely, then wrap tightly with plastic wrap and refrigerate. The next day, simply warm them up in a 375F oven while you take care of the spinach and eggs.)

Prepare the Hollandaise (make up to 1 day in advance):

1. In a small saucepan set over medium-high heat, heat the butter until melted and sizzling hot.
2. Meanwhile, in the bowl of a blender, combine egg yolks, lemon juice, mustard and cayenne. Blend on high speed for about 30 seconds, or until you have a smooth, pale yellow mixture.
3. Reduce the blender speed to low and, with the blender still running, slowly drizzle in the butter in a thin stream until it's completely incorporated into the sauce.
4. Once the butter has all been poured in, run the blender for another 2-3 minutes to ensure the sauce is completely emulsified. If the sauce looks too thick, add more lemon juice, 1 tbsp at a time, until it loosens up to the desired consistency. Season with salt and pepper to taste.
5. (If making in advance, refrigerate the finished sauce, then reheat using 10-second bursts in a microwave or a double-boiler set over simmering water - be careful not to overheat, though, as the sauce will separate if it gets too hot.)

Saute the Spinach:

1. In a large saucepan set over medium-high heat, melt the butter until foamy. Add spinach and cook for 3-5 minutes, or until soft and wilted. Season to taste with salt and pepper. Set aside.

Poach the Eggs (make up to 3 hours ahead):

1. Fill a large pot with 4″ of water, and stir in the vinegar.
2. Bring to a simmer over high heat, then reduce the heat to low. (Do not let the water reach a full rolling boil – you want it to be at a bare simmer for the next step.)
3. Working with one egg at a time, crack an egg into a small ramekin, then gently slide the egg into the hot water. Use a spatula or slotted spoon to gently nudge the egg back into shape as it sets. Repeat the process with a second egg. Cook both eggs for 3-4 minutes, or until whites are set and yolk is still slightly wobbly.
4. Repeat with remaining eggs, poaching no more than two at a time - any more than this will throw off the timing.
5. (If making in advance, plunge the finished eggs into an ice water bath, then reheat in boiling water for 20-30 seconds when ready to serve.)

Assemble the Bennies:

1. Place two boxties on each plate, and top each one with one or two slices of smoked salmon. Arrange the spinach on top of the smoked salmon, dividing evenly between the portions, then top off with poached eggs and a generous amount of Hollandaise sauce overtop. Finish with a dusting of paprika or a sprinkling of snipped chives.