Lightly whisk the eggs, then add the créme fraiche mixed with beaten eggs, oil and milk, flour, baking powder and salt. Mix with a hand-held blender.
Coarsely grate the courgette. Add it gradually to the batter, then add enough soda water to give a thick yet pourable batter. Rest for 30 minutes, if time allows.
Lightly oil a pancake pan, then heat it over medium heat. Using a small ladle, add 2-3 scoops to the pan, allowing them to spread out into circles (or use silicon pancake moulds for perfect disks). When the surface becomes bubbly and it’s no longer liquid, turn them over with a spatula and cook for another minute until golden. Transfer the blinis to a plate.
Repeat with the remaining batter (only re-oil the pan, if necessary).
Serve at room temperature.