Set an immersion circulator in a water bath to 84°C/183°F.
Use a melon-baller to remove pieces of apple and toss with lemon juice and salt (you can core and large dice the apples if necessary). Seal in a vacuum bag with butter then add to the pre-heated hot water bath, ensuring that they are fully submerged. Cook for 40 minutes then remove to an ice water bath to chill quickly.
To reheat, open the bags and pour the apples and butter into a pan and heat over medium heat for 3-4 minutes until warmed through.