Sous-vide Caramelised Onions

There are a variety of recipes online for sous-vide caramelised onions, so when making these I experimented with adding butter and pre-cooking them. The addition of butter seemed to prevent them from developing as much flavour, but the pre-frying is essential to stop the bag ballooning with gas during the cooking and to reduce the crunch of the finished onions. They don't get as dark as caramelised onions in a pan, but the liquid with them is considerably more flavoursome. The bottom left onions in the photo below are the result of this recipe. The others, clockwise from top left, are no-butter without pre-frying, no pre-frying but with butter, pre-frying with butter and, finally, pre-frying without butter.

Ingredients:

• 4 onions
• 1/4 tsp salt

Method:

1. Pre-heat the sous-vide water to 85 degrees Celsius.
2. Fry the onions over a medium heat in a dry frying pan for 15 minutes, stirring occasionally.
3. Use a thick plastic bag, or double bag it, to hold the part-cooked onions and use the submersion method to get all the air out. Some plastic bags don't have strong enough edges to not break during the long and hot cooking of this recipe.
4. Cook the onions for 24 hours.
5. Season with the salt, divide into portions and freeze until required to flavour a recipe.