Peel, trim, and roughly chop onion.
Place metal steamer in a large stockpot and fill with water or vegetable stock to just below steamer base (about 2 cups). Cover, turn heat to high, and bring to a boil. Add onion and replace lid. Steam until onion is tender and beginning to be transparent, about 5 minutes. Add broccoli florets and cover again. Steam about 4 minutes, until broccoli is just tender and bright green.
Remove from heat, and transfer onion and broccoli to high speed blender, leaving 1 cup of steamed broccoli out to blend in later for added texture. To blender, add 2 cups of steam water or stock, miso, coconut oil, nutritional yeast, ume plum vinegar, sea salt, ground coriander, and toasted sesame oil. Blend until completely smooth. Taste and adjust salt as desired. Add remaining 1 cup steamed broccoli and blend until desired texture is reached. Return to pot and cover to keep warm.
Preheat oven or toaster oven to 400ยบ. In a small bowl, whisk coconut oil, soy sauce, ume plum vinegar, cayenne pepper, and chives. Add cashews and sesame seeds and stir to coat completely. Transfer to small baking sheet and spread in an even layer. Roast for 3-4 minutes, or until golden brown.
Prepare soup bowls with cooked rice, miso broccoli soup, sesame chive cashews, chopped peeled broccoli stalks, minced chives, and toasted sesame oil, to taste.