Toss the zucchini with ½ teaspoon salt in a bowl; let stand 10 minutes. Wrap the zucchini in a kitchen towel and squeeze as much liquid as you can.

Meanwhile, heat the butter in a large nonstick skillet over medium-high heat. Add the onion and garlic and cook, stirring occasionally, until slightly softened, about 4 minutes. Add the corn and cook, stirring occasionally, until crisp-tender, about 3 minutes. Set aside.

Whisk the cornmeal, flour, baking soda, ¼ teaspoon salt and pepper in a medium bowl. Whisk the buttermilk and egg in a small bowl or cup. Add the buttermilk-egg mixture and the zucchini-corn mixture to the bowl and stir until just combined.

Heat about ⅛ inch coconut oil in a large nonstick skillet over medium heat. Working in batches, scoop scant ¼ cupfuls of the batter into the oil and use the back of the measuring cup to flatten the scoops. Cook until the fritters are golden brown, 3 to 4 minutes per side. Drain on paper towels and sprinkle with salt if desired. Serve warm or at room temperature. (You can make the fritters up to 2 hours ahead; reheat in a 375 degree oven on a rack set on a baking sheet.)