For the gnocchi: Combine the flour, Parmesan, lemon zest and salt in a medium bowl. Add the ricotta and egg. Mix well with a wooden spoon or your fingers until the dough just comes together, taking care not to overwork the dough, which could cause it to become tough.
Scrape the dough onto a well-floured work surface and pat into a rough square. Cut the dough into fourths with a bench scraper or knife. Gently roll each piece into a foot-long rope about an inch in diameter, flouring as needed to prevent the dough from sticking to the work surface. Cut each rope into 1/2-inch pieces with a bench scraper or knife and set aside.
For the sauce: Heat 6 tablespoons of the butter over medium-high heat in a skillet large enough to accommodate all of the gnocchi without crowding. When the butter begins to foam, add the gnocchi to the skillet and cook, turning as necessary, until they're browned and crisp on all sides, 5 to 6 minutes. Add the sage leaves and cook until crisp, 30 seconds. Season with salt and pepper, then stir in 1/2 cup water, the lemon juice and remaining 2 tablespoons of butter.
Remove from the heat and sprinkle in the Parmesan while turning the gnocchi. Let the sauce ingredients emulsify and form a silken coating, about 1 minute. Add more water if the skillet looks too dry. Spoon the gnocchi and sauce into shallow bowls and serve immediately.