uick soak the cashews: Bring a few cups of water to a boil (I use my tea kettle for this). Add 1 1/2 cups cashews to a measuring cup and pour the boiling hot water on top to cover. Let them soak for 5 minutes, or 1 hour if you don't have a high powered blender.
Drain cashews and add to high speed blender. Now add 3/4 cup fresh water, apple cider vinegar, lemon juice and salt. Blend until very smooth.
Pour the cashew mixture into a bowl and stir in in the unsweetened yogurt. This is optional, but it brings the sour cream to another level in terms of texture and flavor.
Use immediately, or transfer to a small covered container and refrigerate. It will thicken up as it chills. The sour cream will keep for about a week. You may also freeze this if desired.