Heat the oil in a large skillet, or saucepan over medium heat. Once hot, add garlic and cook for 2-3 minutes. Rinse spinach, drain and add to the skillet. Cook for about 5 minutes, stirring regularly until spinach is wilted.
Stir in the coconut milk and add lemongrass, galangal, mushrooms, coconut sugar, soy sauce, and green chili if using. Let simmer gently over low-medium heat for about 10 minutes, do not bring to a high boil.
Remove from heat and discard the galangal. Transfer to a blender and blend until smooth and creamy. Serve immediately with a drizzle of lemon juice, and topped with the cashew tempeh and fresh basil. Soup will keep for up to 3 days in the refrigerator.
Cashew Tempeh
Cut the tempeh into 1/2-inch cubes. Place the cubes in a baking dish, or large bowl. Set aside.
Add the soy sauce, maple syrup, sesame oil, cashew butter, sriracha, and garlic to a small bowl and mix until well combined. Pour over the tempeh and stir to coat. Let marinate at least 1 hour or overnight in the refrigerator.
Heat a tablespoon of oil in a non-stick skillet over medium heat. Once hot, add tempeh cubes, leaving the remaining marinade in the bowl.
Sauté for 3-5 minutes, stirring regularly to cook the tempeh evenly. Deglaze the skillet with the remaining marinade and remove from heat. Serve with the curried spinach soup.
Notes
You can blend the galangal with the other ingredients if you want. Just be aware: if you don't have a powerful blender you may feel some fibers in the soup.
This soup is already thick, but it might thicken a bit more in the refrigerator because of the full-fat coconut milk, this is normal and it will become thinner/creamier once reheated. If you prefer a thinner soup, add 1/2 cup vegetable broth when blending the soup.