Pour 4 cups whole milk into a heavy-bottomed pot. Heat over medium, stirring occasionally to prevent scorching, until the milk reaches 185°F on an instant-read thermometer.

Remove the pot from heat and let the milk cool to 110°F. To speed cooling, set the pot in a cold water bath and stir occasionally while monitoring the temperature.

In a small bowl, whisk 3 tablespoons plain yogurt with a small amount of the cooled milk until smooth. Stir this yogurt-milk mixture back into the rest of the cooled milk until evenly combined.

Cover the pot and keep it at about 110°F for 5–10 hours, undisturbed, until the mixture has thickened. (Use an oven with the light on, a yogurt maker, or another warm spot that holds roughly 110°F.)

Line a fine-mesh sieve with cheesecloth and set it over a bowl in the refrigerator. Pour or spoon the yogurt into the lined sieve and refrigerate for 4 hours to drain whey and achieve Greek yogurt consistency.

After draining, transfer the strained Greek yogurt to a covered container and store it in the refrigerator.