Generously rub the salt on the chicken and let it sit, uncovered, in the fridge overnight on a plate.
Remove from the fridge and stuff the cavity with the onion, half the thyme and lemon.
Gently use your fingers to separate the skin on the breast from the meat and put a tablespoon of butter under each side.
Place the chicken in a bag with the smashed garlic cloves and remaining 2 tablespoons of butter and thyme.
Use a vacuum sealer to seal the bag (or use the water displacement method explained above).
Heat a water bath to 150F degrees and place the chicken in. Cook for 6 hours.
Remove the chicken from the sous vide cooker and immediately place in an ice bath for 10 minutes to cool the chicken.
While the chicken is in the ice bath, heat a broiler on high.
Remove the chicken from the bag and place in a large baking dish under the broiler.
Broil for 5-10 minutes, watching closely to ensure the skin doesn't burn.*
Lemon Buerre Blanc:
Bring the wine to a simmer over medium heat and stir in half the butter.
Simmer, over medium low heat, for about 5 minutes.
Stir in remaining butter and add salt to taste. Simmer for 2-3 minutes.
Remove from heat and stir in lemon juice.
*You can also use a large high powered kitchen torch, sear it in a pan, or finish over a hot grill to crisp the outside.
It's important to stuff the cavity of your chicken. As you can't get all the air out of the cavity when it's vacuumed sealed, the stuffing in the cavity helps hold the temperature for your chicken as it holds onto the heat. Plus the lemon and onion adds extra flavors. If you have fresh herbs on hand, you are more than welcomed to stuff that in the cavity as well.
When you leave the chicken uncovered in the fridge overnight with salt, it helps draw out the moisture so when you finish the chicken off in the oven, the skin crisps up beautifully so don't skip this step!
Measure twice, cut once when it comes to the vacuum seal. Since a whole chicken is quite large, double check that your packaging can cover the entire chicken.