Peel and chop the onion
Put the oil into a heavy-based skillet
Cook the onions very slowly in the oil for around 50 minutes. Stir occasionally to prevent burning
After 30 minutes, if you want to help caramelise the onions stir through a teaspoon of sugar
If the mixture starts to dry out, then add a tablespoon of water
Once the onions are soft and caramelised, take them out of the pan and keep warm. Leave as much oil in the pan as possible
Season the pigeon breasts with salt and pepper
Turn the heat up so that the oil is just sizzling and add the pigeon breasts to the pan
Cook on one side for 2 minutes, pressing down gently with a spatula or the back of a wooden spoon
Flip the pigeon breasts and cook on the other side for a further 1-2 minutes
Take out of the pan and allow to rest while you start to construct the sandwich
Butter the bread if you like.
Spread the mustard over the bread
Add half the rocket
Add a teaspoon of caramelised onion
Slice the pigeon breast and arrange it on top of the onion
Top with the remaining rocket and another teaspoon of onion
Finish with the remaining slice of bread.