Combine the milk, cream, and salt in a medium saucepan over medium heat. Stir the mixture regularly, to prevent scorching the bottom of the pan, until the milk becomes steamy and frothy. You don’t want it to do much more than simmer gently.
While the milk mixture heats, set a colander in a large bowl and line the colander with a very clean dishtowel or a couple of layers of cheesecloth. The cloth should hang generously over the sides of the colander.
When the milk reaches the foamy, frothy point, add the lemon juice or vinegar. For a creamier, moister ricotta with small curds, remove the pot from the heat immediately and let it sit for about 10 minutes. If you’d prefer a drier, denser ricotta with larger curds, keep the mixture on the heat, very gently simmering, for 1 to 2 minutes. In either case, you’ll see the white curds begin to separate from the thin-looking, watery, yellowish whey.
Strain the mixture into the lined colander and let it drain until the consistency looks good to you. (If you want to eat it, it looks good!) You may feel as though a lot of liquid is draining off, and you’re right. Make sure the bottom of the colander doesn’t touch the the whey; if it does, put a fresh bowl under the colander. But don’t toss the whey! Save it for up to 3 days and use it to make bread, cake, smoothies, creamy soups, or porridge—or even cook a grain like rice or quinoa in it.
For a small-curd ricotta, the mixture may still look pretty moist when you decide to pack it into a container and stick it in the fridge, and it may take up to an hour to look that way; a larger-curd ricotta may take as little as 5 minutes. Keep in mind that the cheeses will firm up slightly in the refrigerator. Ricotta will keep for about a week in the fridge.