Heat 1 tablespoon of the olive oil in a small saucepan over medium heat and add the onion, salt and the prawn heads and shells. Stir until the heads and shells are bright orange and fragrant.
Add the remaining oil and cook over very low heat, uncovered, for a further 20 minutes, stirring occasionally.
Pour the oil through a fine sieve to remove any solids. Your prawn oil is ready!