1/4 cup yogurt with live active cultures (also known as khumsah) *
Heat the milk very carefully over medium-low heat so that it doesn’t scorch or boil over. Stir it every few minutes and keep a very close eye on it.
As soon as it starts to simmer, take it off the heat and pour it into the container you’re going to store it in (either glass or ceramic). Or you can leave it in the same container you heated it in, as long as it’s a non-reactive metal and you have avoided scorching the bottom.
Let it cool down a bit, checking periodically to make sure the milk doesn’t cool down too much. It should still feel warm, but should be cool enough that you can stick your finger in the milk without burning yourself. Once it’s at this point, add the khumsah to a small bowl and combine it with 1/4 c of the warm milk.
Add this mixture back into the container, mix it together, and cover it with a loose-fitting lid or plastic wrap.
Wrap the whole thing in a blanket. This will keep it insulated so that it keeps some of the residual heat from the milk.
Store the blanketed container in the oven and put a note on the control board so no one turns it on while your yogurt is inside.
Let it ferment anywhere between 8 to 24 hours, depending on how sour you want it to be. It will become more sour as it ages. 12 hours is usually perfect.
Once you’re happy with the consistency, put it in the refrigerator to let it chill.
Once it’s chilled, put a couple of layers of undyed paper towels or clean tea towels (washed without dryer sheets, fabric softener, or fragrance) on top to soak up some of the whey. Discard after they absorb as much liquid as they’ll hold.
Enjoy the mesta as is, or hang it with cheesecloth to turn it into Greek yogurt (hung for about 4 hours) or labneh (hung for about 12 hours).
* The khumsah can be from a homemade batch of yogurt, or a scoop of store-bought yogurt.