Chop 1 onion. Mince 4 cloves garlic. Chop ½ cup basil leaves.
In a medium pot, heat 2 tablespoons olive oil over medium heat. Add onion and sauté for 5 minutes, until translucent. Add garlic and red pepper flakes; sauté 1 minute until fragrant. Add white wine vinegar and cook until evaporated.
Add basil, cashews, canned tomatoes and their liquid, broth, kosher salt, and black pepper. Bring to a boil, then reduce heat and simmer 20 minutes. When done, use an immersion blender to puree the soup for several minutes until creamy and the cashews have fully incorporated (alternatively, puree in a blender).
Meanwhile, toast the bread slices and use small, star-shaped cutters to cut out star shapes. Chop the chives.
Serve soup warm with star toasts and chopped chives.