Heat oil in a frying pan and fry the sliced onion until softened.
Whisk the crème fraîche and chopped chives in a bowl and season with salt and pepper.
Combine whisked duck eggs, halved potatoes, spring onions, fried onions, and cheese in a mixing bowl.
Heat 30ml of rapeseed oil in a non-stick frying pan and pour in the egg mixture.
Stir the mixture with a spatula until the egg starts to set, then let it brown slightly.
Place a cake stand over the pan, flip to invert the tortilla, and slide it back to finish cooking.
Cut into portions and serve topped with a spoonful of crème fraîche and chive flowers.