Step 1 Spoon ¼ cup of the milk into a bowl and stir in the yogurt.
Step 2 In a saucepan, bring the remaining milk to a boil. Remove from heat and set aside until the milk registers 100 degrees using a thermometer, 15 to 30 minutes; a skin will form on the surface.
Step 3Using a table knife, make a small opening in the skin and carefully pour the cheese mixture into the milk in the saucepan. Cover the pot with a kitchen towel and transfer to an oven. Turn on the light and close the oven door. Set aside for 16 hours, no more than 18 hours.
Step 4Using a skimmer or slotted spoon, lift off the skin and discard it. Carefully ladle the cheese into a sieve lined with a double layer of cheesecloth and refrigerate until much of the whey is drained and the cheese is thickened to a labneh-like consistency, about 4 hours (the longer you drain, the thicker the cheese). Transfer the cheese to a bowl and serve, or refrigerate until ready to serve; the cheese will keep up to 4 days, refrigerated. If desired, flavor the cheese with chopped thyme and lemon zest.