For the red onion and plum relish, heat the oil in a frying pan. Add the red onion and plums and fry over a high heat. Add the remaining relish ingredients to the pan and simmer for five minutes until softened.
Spoon the relish into a small serving bowl and leave to cool while you prepare the omelette.
For the omelette, melt the butter in a small frying pan.
Mix the beaten egg with the chopped ham in a small bowl and season well with salt and freshly ground black pepper.
Add the egg mixture to the pan and mix with a wooden spoon until it starts to set around the edge of the pan. Leave to cook over a low heat until it has completely set. Carefully flip the omelette out onto a plate and slice into wedges.
Serve the omelette with the relish on the side, garnished with chopped parsley.
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