Place the cashews in a bowl and cover them with boiling water. Soak for 1 hour.
Add olive oil to a large skillet over medium heat. Toss in the chopped onion and garlic, and let it cook until fragrant, about 3-5 minutes.
Add in one tablespoon of flour. Using a wooden spoon, stir the flour and cook until it browns slightly, about 3 minutes.
Add frozen spinach, plant-based milk and salt. Give it a good stir, and bring the mixture to a boil. Reduce the heat to low and let it cook for about 10 to 15 minutes, until the spinach has melted and the mixture begins to thicken.
Let it simmer for another 5 minutes, stirring occasionally.
Meanwhile, drain the cashews and add them to a blender with the plant-based milk. 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.
Serve as is or choose from one of the toppings mentioned above.