Finely chop the onion and garlic.
Melt the butter in a heavy-bottomed pan/pot over medium-high heat (8 out of 14 on my stove).
Add the onion, garlic, and rice and sauté it for about 10 minutes. Stir well while the onions soften and the rice absorbs the fat.
Put it directly into a vacuum bag and add the stock and salt.
The bag should not be vacuumed, but you should simply press out as much air as you can yourself and then seal/weld the bag. If you have a chamber vacuum sealer, you can of course vacuum seal it directly.
Put it in your sous vide water bath and cook it for 1 hour and 10 minutes. Feel free to take the bag out of the water bath twice to massage the liquid a bit.
Take the bag out of the water bath and directly into a bowl.
Add the freshly grated parmesan and butter.
Stir and season with salt, pepper, and possibly lemon juice/zest.
Serve immediately.