In a small iron skillet, toast the first seven spices, one at a time, over high heat. Remove each spice from the heat once it begins to change color and the air has a definite spicy odor.
Once the spices have cooled sufficiently, mix them and grind them into a fine powder. Set aside.
Heat the peanut oil in a stockpot until very hot. With the lid of the stockpot in one hand, pour the brown mustard seeds in the oil and immediately cover. When the popping subsides, remove the lid (it should smell like popcorn).
Add the garlic, give the pot a shake, then add the onion and immediately stir to mix. Lower the heat, cover, and let cook for 5 minutes, or until the onion is golden. Add the minced ginger and the ground spices and mix well.
Place the peanut butter in a large bowl. Add the honey and whisk until blended. Add the boiling water, 1 cup at a time, whisking after each addition until evenly blended. Pour the contents of the bowl into the pot and mix well. Cover and let the soup reach a boil over medium heat. Stir occasionally so the onions and spices don't burn on the bottom of the pot. Add the buttermilk and stir well. Cover and let simmer for 15 minutes over low heat.
Once the soup reaches the consistency of runny pancake batter, remove it from the stove. Serve immediately with thinly sliced green onions.