Make the ricotta filling. Place ¾ cup ricotta cheese in a small mixing bowl with 3 Tbsp grated Parmesan, 1 egg yolk, 2 Tbsp chopped sage, 1 tsp fresh lemon zest, ½ tsp Kosher salt, and ¼ tsp black pepper, then stir until well mixed. Transfer the filling to a medium Ziploc bag, pressing down into the corner. Set aside until needed.
Stuff the zucchini blossoms. Cut off the tip from one corner of the bag, then insert carefully into the squash flower. Stuff each with about 1-2 Tbsp of filling, depending on the size. You don't want them overflowing. It's ok if the leaves rip a little, simply press them gently around the filling. Place stuffed squash blossoms in a refrigerate to firm up for 1 hour.
Preheat oil. Before you begin dredging, preheat enough neutral oil in a cast iron skillet over medium-high heat to coat the bottom with 1-inch of oil. Look for between 375-400°F / 190-205°C.
Dredge the squash blossoms. In a shallow bowl, whisk together ¾ cup flour and ¾ tsp Kosher salt. Pour 10oz of club soda on top, then whisk until there are no lumps. Dip the stuffed squash blossoms in batter, twisting to coat fully, then allow any excess to drip off. Place immediately in the hot oil. Best to test one first to see whether the oil is too hot (burns + cooks quickly) or not hot enough (oily texture).
Fry the squash blossoms. I like to fry 3 or 4 at a time. Cook for 1-2 minutes per side, using tongs to flip as needed. Once golden brown, transfer to a paper towel lined wire cooling rack. Sprinkle immediately with a pinch of flaky sea salt and additional lemon zest, if using. Repeat with remaining batches. Cool slightly, then serve!