Wash the fennel and remove the stalk. Cut the fennel so that narrow strips emerge. Cut the onion into rings and add to the fennel. Season with olive oil and aceto, salt and pepper and let steep for 5 minutes.
Peel oranges. The white skin must be completely removed, the best way to cut the skin off properly. Then cut out the orange fillets.
Add the resulting juice to the fennel.
Put the fennel on a plate and place the orange fillets on top of the fennel.