Preheat oven to 425F. Scrub potatoes clean and arrange in a single layer on a medium sheet pan. Prick potatoes all over with a fork.
Roast potatoes at 425F 50-60 minutes until potatoes are tender when pierced with a fork. Slice each potato in half horizontally to allow steam to escape and allow potatoes to cool completely.
Place spinach in a food processor and pulse until spinach is finely minced. Set aside.
Remove skins from potatoes and run potatoes through a ricer or a food mill onto a large cutting board. Make a well in the middle of potatoes and drizzle in egg and sprinkle with salt, minced spinach, and flour. Use a benchscraper to gently fold and toss mixture. Keep gently folding and kneading dough until a smooth ball forms.
Divide dough into 8 pieces and sprinkle a light dusting of flour on cutting board. Gently roll each portion of dough out into an 8-inch rope, dusting surface with additional flour as needed. Use a benchscraper to cut rope into 1-inch pieces.
Dip the tines of a fork in flour and gently indent each gnocchi, rolling back and forth and dipping fork in additional flour as needed.
Bring a pot of water to a boil and lightly salt. Working in batches, boil gnocchi 2-3 minutes until the float to the top of water. Use a slotted spoon to remove gnocchi and transfer to a plate.
In a large skillet over medium heat, melt butter. Add sage leaves and cook 2-3 minutes, turning occasionally until sage is crispy and butter is golden-brown. Remove sage and drain on paper towels.
Add gnocchi to sage butter in pan and toss 1-2 minutes over medium heat until gnocchi is evenly coated. Remove gnocchi from heat and season with salt and pepper to taste. Top gnocchi with freshly grated Parmesan and crispy sage leaves. Serve warm and enjoy!