Heat the oil in a 4 or 5 quart Dutch oven or soup pot over medium heat until shimmering. Saute the onions and sweet potatoes until the potatoes are golden with brown edges, about 8 to 10 minutes. Add the garlic and ginger and cook for a minute or two, until fragrant. Stir in the tomato paste and mix thoroughly with the vegetables. Add the crushed tomatoes and vegetable broth. Increase heat to medium-high and let the soup come to a gentle boil.
Spoon the nut butters into heat-proof bowl. Ladle 1 cup of the hot soup into the bowl and stir, gently at first, until the liquid combines with the nut butters, then with more vigor, until the mixture is very smooth and loose.
Pour the nut butter mixture into the soup, and add the collard greens and sriracha sauce (1 tablespoon for gentle heat; 3 tablespoons for a more assertive heat). Reduce heat to medium-low, and allow the soup to simmer for 15 minutes.
Ladle into bowls and garnish generously with the scallions and nuts.