Set your sous vide immersion cooker to 190ºF/88ºC and allow the water to heat up.
If keeping the garlic whole, cut the top off the garlic head. If not, separate and peel each clove.
Add the garlic to a jar, layer with thyme, if using, and top up with fat. Seal the jar. (see tip above)
Submerge the sealed jar in the heated water and cook for 4 hours.
Close to the end of 4 hours, prepare an ice bath. Remove the jar from the hot bath and allow cool at room temperature before submerging in an ice bath for 15 to 20 minutes. Immediately refrigerate. If cooking in a vacuum bag, immediately submerge in the ice bath after cooking, then transfer to a jar and refrigerate.