- Place porcini in a saucepan, cover with water and set over high heat. Bring to a boil, remove from heat and allow to sit for 10 minutes until softened. Remove porcini from water and finely chop. Set aside.
- Place potato in a saucepan, cover with water and set over high heat. Bring to a boil, reduce heat to medium and allow to gently boil until soft, about 25 minutes.
- While the potatoes are cooking, make the stuffing. Place 2 tablespoons olive oil in a frypan and set over medium heat. Add onion, porcini mushrooms and finely chopped sage and fry until golden. Remove from heat and set aside.
- Drain potatoes and mash. Pass through a drum sieve into a bowl. Add flour, egg yolk and ¼ teaspoon salt and stir to combine.
- Lay a piece of plastic wrap on a work surface. Place mashed potato on top and spread to 1 cm thickness. Add stuffing mix and spread. Roll tightly and secure ends to make a firm roll.
- Bring a saucepan of water to a boil over high heat. Add potato roll and cook until set, about 10-12 minutes. Remove from water, unwrap plastic and slice 2 cm thick.
- Place remaining 100 ml olive oil in a frypan and set over medium heat. Add gnocchi slices and fry until lightly brown on both sides. Remove from frypan and set aside.
- Add remaining sage leaves to the frypan and cook until crispy. Season and remove from heat.
- To serve, place gnocchi slices in a bowl with crispy sage leaves.