Feta is a Greek cheese, originally made with sheep's milk or a mixture of goat's and sheep's milk. Most feta sold in supermarkets is now made from cow's milk. Feta is a salty, crumbly cheese that is delicious sprinkled on salads and incorporated into pasta and many other hot dishes.
Pour the milk in a large saucepan and heat it to 85 degrees. Add the yogurt and whisk until the yogurt is completely incorporated into the milk. Remove the mixture from the heat and allow it to set for two hours.
Dissolve the rennet tablet in 3 to 4 tbsp. of cool water and whisk it into the milk. Stir the mixture constantly for at least 5 to 6 minutes. Allow the mixture to set for 1 to 2 hours, until the curd is firm enough to break cleanly when cut with a knife. Use a long knife to cut the curds into dice-sized cubes and allow the cubes to firm up for 10 minutes.
Give the curds a gentle stir and if any pieces are larger than dice, cut them. Allow them to sit for a half hour, stirring occasionally.
Drain the whey from the feta by pouring the mixture through a colander lined with muslin or cheesecloth. Tie the corners in a knot and let the curds continue to drain for about 5 hours.
Place the feta curds in a shallow rectangular container with a tightly fitting lid so that it's only about an inch in thickness. Cover the container and refrigerate the feta for 90 minutes. Take the feta out of the refrigerator and cut it into cubes about 1 inch square.
Make a brine by dissolving 1½ cups of salt in a quart of warm water, and chill it thoroughly. Soak the feta cubes in the cold brine in the refrigerator for 6 to 30 days. Feta that is aged longer will be more crumbly.
Remove the feta from the brine and pat it dry with paper towels.
Refrigerate the finished feta in an airtight container.