We peel the onions and we cut them into quarters.
We put salt and pepper all over the salmon fillets.
In a shallow saucepan, we sauté the onions until they start to brown with half of the olive oil and butter. We finish sautéing by pouring the orange juice; we add the vinegar and we boil until the liquids evaporate.
In a frying pan, we sauté the salmon fillets from both sides until they start to brown with the rest of the olive oil and butter.
We serve the marmalade in individual plates with the salmon fillet on top.