To make the sponge: In a small bowl, combine the water, flour and yeast. Mix well and set aside in a warm place until yeasty-smelling and covered in very small bubbles, about 45 minutes.
In a medium saucepan, cover the potatoes with cold water. Bring the water to a boil over high heat, add 1 tablespoon of sea salt. Reduce the heat to a simmer, and cook until the potatoes are tender, 20 to 25 minutes. Drain and mash the potatoes with a fork. Allow them to cool.
In a small bowl, whisk together the eggs and olive oil until thoroughly combined. In a bowl, combine cooled potatoes with the sponge, flour, sage, lemon zest, pepper, and 2 teaspoons of salt. Mix to evenly distribute the ingredients. Add the egg mixture to potato mixture, and stir until thoroughly combined. Use a rubber spatula to scrape down the sides of the bowl. You should have a thick, sticky batter. Add a bit more flour, if needed. Cover the bowl with a towel and set aside in a warm place to rise until doubled in size, for 1 1/2 to 2 hours.
Heat the oil in a deep-fryer to 375 degrees F.
Scoop 1 tablespoon of the risen batter at a time, and use another spoon to scrape it into the hot oil. Don't crowd the fryer. Fry fritters until golden brown, 2 to 3 minutes, turning them over occasionally. Drain the fritters on paper towels to absorb any excess oil. Sprinkle with sat and serve warm.
Entertaining Tip: You can fry up a couple of batches of these an hour before your guests arrive and simply heat them again in a 300F oven before serving. Or keep warm in a 250F oven after making them. The fritters are great alone but they are equally great with a couple of dipping sauces.