Put milk into a Dutch oven, place on the middle rack of your oven and turn it on to 225ºF. Bake uncovered for 6-8 hours, or until the top crust turns golden brown color.
Remove the crust, and eat it - it's amazingly delicious!
Cool baked milk in refrigerator in order for the fat to harden so you can remove it (This step isn't necessary but it makes the texture of ryazhenka creamier and more appealing).
Strain through a fine strainer into a glass jar.Add a tablespoon of sour cream (or kefir) for every couple of cups of baked milk (the amount you add doesn't matter that much, it will ferment even with a very small amount).
Cover tightly, and leave at room temperature until it thickens (anywhere from 4 to 12 hours). Higher temperature will speed fermentation. The final product will be thick enough so that a crust (if you were to put it back in) would stay on top without sinking.
Ryazhenka keeps well in the fridge for a 2-3 weeks.
If you prefer pourable consistency - shake it well. Unshaken it has a thick yogurt-like texture.
