After the cashews have soaked for at least 8 hours, drain them and place them in a food processor or high speed blender. Blend until mostly smooth, scraping down the sides a few times if needed.
Add water and gelatin to a small saucepan over low heat. Stir until the gelatin has completely dissolved, then add this mixture to the blender along with the lemon juice, salt and cream of tartar. Blend until well combined and smooth.
Pour the cashew mixture into an 8x4 parchment-lined loaf pan and refrigerate overnight or until the paneer has firmed.
Cut the paneer into cubes or desired shape and store in the fridge until ready to serve.