Ricotta cheese made by adding milk to the whey left over from making Greek yogurt, then boiling and straining it.
Add whey and milk to a pot and bring to a boil. Add a little salt at this point.
When bubbles start to rise, add lemon juice and continue boiling without stirring until it curdles like soft tofu.
Once it curdles, turn off the heat and let it cool.
Strain the whey through a cheesecloth and squeeze the curds tightly to remove excess liquid.