Add soaked (or boiled) cashews into a high-speed blender. Start the blender and add water slowly, until the cashews are blending smoothly and turning into a thick cream. Depending on how hydrated your cashews are, you might need a little more or little less water than 1 cup.

Transfer the cashew cream into a glass bowl. Add the probiotic capsule and stir with a wooden spoon (avoid using metal utensils as metal can react with the probiotics). Add the salt and stir again.

Cover the bowl with a piece of cheesecloth (or an airtight lid for a quicker culturing). Place the sour cream in oven for about 24-48 hours. (The culturing time will depend on many factors, such as the temperature in the oven, the temperature in the house, if you're using an airtight lid, how tangy you like your sour cream, etc). 

Stir the sour cream after 24 hours and taste. If it's not tangy enough, cover it, and put it back in the oven. You can leave the sour cream in the oven for about 36 hours at 80°F (27°C). 

If the sour cream isn't tangy/sour enough for your taste, add some lemon juice.

Transfer the sour cream into a glass jar with an airtight lid. Put it in the refrigerator for 6 hours to chill. The sour cream will continue to thicken in the refrigerator. You can always add a splash of water and give it a stir for a thinner consistency.

Store leftover sour cream in an airtight container in the refrigerator for up to 5 days. For longer term storage, freeze in an airtight container for up to 3 months.