Preheat the oven to 220°C (430°F) or 200°C (390°F) for fan-forced ovens. Line a large baking tray with foil or baking/parchment paper. Wash and dry the potatoes, then use a fork to pierce each potato everywhere (around 4 to 5 times). Bake the potatoes for 40 to 60 minutes, just until the potatoes are tender. A skewer should pierce through the potatoes effortlessly, but don't over-bake the potatoes as hard, thick skin will form around the potatoes.
Allow the potatoes to cool for 10 minutes or cool enough to handle. Peel the potatoes and cut them in half. Use a potato ricer to press the potato halves into a large bowl. If a ricer is not available, you can grate the potatoes using a box grater. Don't put the potatoes in a blender or food processor as they'll turn into a gluey texture.
Place the flour on a clean workbench, then mix in the salt and pepper. Make a well in the centre of the flour, then add the potatoes, egg and ricotta. Mix the ingredients together with your fingers until well combined and gather them together to form a dough. Gently knead and fold the dough a few times but don't overwork the dough as you may end up with tough gnocchi. The dough should be soft but not sticky.
Cut the dough into 6 to 8 pieces and roll each piece into a rope. Cut out approximately 2 cm (3/4 in) pieces from each of the ropes. Flour a finger and make a visible dent in the middle of each gnocchi (or roll them against the back of a fork) Lay the gnocchi in a single layer on a lightly dusted tray, then lightly sprinkle some more flour on top of them. Let them rest uncovered for 30 minutes before cooking.
Fill in a medium saucepan 3/4 full with water, then add a tablespoon of salt into the water. Place the pan over medium-high heat and bring the water to a boil, then add a batch of gnocchi (cook gnocchi in batches to prevent them from sticking). As soon as the gnocchi floats, it's done, so remove with a slotted spoon or strainer and place them in a single layer on a plate.
Heat oil in a large frying pan over medium-high heat, then add a batch of gnocchi and place them in a single layer in the pan. Cook the gnocchi on each side for 1-2 minutes or until they have golden crusts. Transfer the fried gnocchi onto a plate.
Wipe the same frying pan clean with paper towels. It's okay if there's a small amount of oil remaining. Place the pan over medium-low heat and add the butter. Swirl the pan to help the butter melt, then leave it untouched. Once the butter smells nutty and the milk solids (dark bits) have sunk to the bottom, add in the garlic, sage leaves and salt. Cook for a minute, or until sage leaves are crispy, then add in the gnocchi. Cook for 2 to 3 minutes, stirring gently or until the gnocchi is heated through. Have a taste and add more salt or pepper if needed. Place the Gnocchi onto plates, then grate a generous amount of parmesan cheese over the top.