Place the potatoes in a saucepan of boiling water and simmer for 20 minutes. Drain, return to the pan over a low heat and, shaking the pan, allow the excess moisture to steam off. Mash the potatoes until smooth.
Cook the spinach in a large pan over a medium heat with no added moisture, mixing constantly until wilted and soft. When cooked, place in a clean kitchen cloth. Gather the corners together and twist tightly to squeeze out every last drop of moisture, then finely chop the spinach.
Place the mashed potatoes, spinach, flour, egg yolk and seasoning in a large bowl and mix until evenly blended and a soft pliable dough forms. Dust your hands and a board or tray with flour. Take small nuggets of the dough, roll it into balls and place them on the board. You should end up with about 60. Using a fork, gently flatten each ball, allowing the tines of the fork to make indentations across the top.
Heat a large pan of water just to boiling point then reduce the heat. To the simmering water add just enough gnocchi so that they have room to roam around, and cook for 2 minutes or until they rise to the surface. Remove and drain well, and cook the next batch.
Place a layer of the cooked gnocchi in the dish, then crumble in some of the blue cheese. Continue to layer the gnocchi and cheese, adding seasoning and the remaining butter cut up into little pieces as you go.