First, soak the cashews. You can do this overnight, for at least 6 hours, or use the quick-soak method for 30 minutes. To soak overnight or for 6 to 8 hours, place the cashews in a bowl, cover with water, and let them sit at room temperature. Drain before using. For the quick-soak method, place the cashews in a saucepan, cover with water, and bring to a boil. Turn off the heat, cover the pan, and let them sit for 30 minutes. Drain before using.
Warm the olive oil in a soup pot or Dutch oven over medium heat. Cook the onion for about 5 minutes until translucent. Stir in the minced garlic, oregano, parsley, salt, and red pepper flakes. Cook for 1 to 2 more minutes until fragrant.
Add the diced tomatoes with their juices and soaked cashews to the pot. Increase heat to bring to a boil, then reduce heat to maintain a simmer. Cover and cook for 15 to 20 minutes.
Uncover the soup and turn off the heat. Use an immersion (handheld) blender to puree the soup in the pot. You can also use an upright blender. Transfer the soup contents in batches to the blender, pouring each blended batch into a large bowl before blending the next batch. Be sure to take safety precautions: do not fill the blender all the way and let steam escape either by leaving the plastic cap in the middle of the lid off or covering the blender with a dish towel instead of the lid.
Ladle the soup into bowls, top with basil, and enjoy!
Leave a rating or review by tapping the stars on this recipe card (above) or in the comments section (at the end of the post).