In a large mixing bowl, grate the zucchini through the large holes of a box grater.
Sprinkle the grated zucchini with salt. Let sit for 10 minutes to draw out excess moisture. Then use a clean towel or cheesecloth, and place the shredded zucchini in it to squeeze out as much excess water as possible.
In a large bowl, whisk the egg. Stir in the sourdough discard, flour, Parmesan (if using), green onions, garlic, pepper, lemon zest, flour, and baking powder.
Add the drained zucchini and mix until well combined.
Heat 1–2 tablespoons of oil in a skillet over medium-high heat. Scoop about 2 tablespoons of batter per fritter into the hot pan, flattening slightly. Cook for 3–5 minutes per side, or until golden and crisp.
Drain on a paper towel or cooling rack. Serve warm with sour cream, Greek yogurt, or your favorite dipping sauce.