Grate the potatoes into a bowl. Squeeze to remove excess water if needed.
In a large bowl, combine the cooled mash potato and add the grated raw potato and half of the sage.
Season well with salt and pepper and stir to create a soft dough. Add some flour to help stiffen the mixture if needed.
Heat some oil in a pan over a medium to low heat, form and shape thick discs of the potatoes mixture and lightly press on the pan to make sure it is sticking together.
Cook for 3 -4 minutes until the base of the boxty is golden brown. Flip over and add the butter and the rest of the sage. Continue cooking for a further 3-4 minutes depending on the thickness until the boxty are piping hot in the middle, basting with the foaming butter.
Remove from the heat and serve immediately with slices of smoked salmon, poached or fried eggs, wilted spinach and sprinkle of sea salt and extra fresh sage if desired.