Place the cashews in a bowl and cover them with boiling water. Soak for 1 hour.
Bring a large pot of water to a boil and add the spinach. Cook until bright green and wilted, about 2 minutes. (If using frozen spinach, it might take a few more minutes to thaw.) Pour the spinach into a colander and apply pressure to the top to remove all of the liquid. Transfer the spinach to a cutting board and coarsely chop it. If there’s still a lot of liquid coming out, place a towel over the top and apply pressure to soak up any remaining water.
Heat the olive oil in the same pot over medium-high heat. Add the onion and garlic and sauté for 5 minutes. Add the spinach and cook for 5 minutes more.
Meanwhile, drain the cashews and add them to a blender with the water, lemon juice, vinegar, salt, and nutmeg. Blend on high for about 1 minute, until very smooth.
Stir the cashew cream into the spinach mixture, then remove from the heat. Serve immediately.
Make It Ahead
This dish can be made up to 3 days in advance and stored in an airtight container in the refrigerator. Return the mixture to a skillet over medium-low heat and stir until heated through.