Start by placing butter at the bottom of your vacuum bag.

Add the spinach leaves to the bag and sprinkle in the salt.

Vacuum seal the bag and place it in your sous vide water bath.

Cook the spinach for 15 minutes at 85 degrees Celsius (185 degrees Fahrenheit).

Make sure to place something heavy on top of the spinach as it is very light and will float to the top. I use a rack that came with my water bath typically used for steaks. You can place it so the spinach cannot reach the surface. An alternative is to use a plate or a small ovenproof dish.

Remove the spinach from the bag and serve immediately.