Preheat oven to 250° and set a baking sheet in it. Mix flour, baking soda, and 1 tsp. salt in a medium bowl. Add egg and 3/4 cup water and whisk until smooth. Stir in zucchini, onions, and basil shreds.
Heat an 8-in. nonstick frying pan over medium-high heat, add 1 tbsp. oil, and swirl. Spoon in one-quarter of batter and quickly spread even. Cook until underside is deep brown, 2 minutes; flip. Add a little oil if pan looks dry; brown second side. Transfer to oven. Make more pancakes the same way.
Quarter pancakes, garnish with basil sprigs, and add more salt if you like.