Blanch the collards: Bring a large pot of salted water to a boil; prepare a large bowl of ice water. Add the collards to the boiling water and cook until bright green and wilted, about 1 minute. Shock in the ice water, then drain and press as dry as possible between paper towels.
Cook the base: In a large bowl, whisk together and combine each ingredient in order and until fully blended before adding the next, starting with the sour cream and then moving on to the heavy cream, flour, allspice, harissa, a nice pinch of salt and a few hefty grinds of black pepper. Make sure to whisk well to combine everything.
Place a large pot over medium heat and add the oil. Add the sausage and cook, breaking up the chunks with a wooden spoon into bite-size pieces, until browned, 5 to 8 minutes. Add the onion and garlic and cook, stirring often so the garlic doesn't burn, until the onions are tender, 5 to 7 minutes. Add the cream mixture. Add the collards, stir well, cover and steep on medium heat until tender, 20 to 30 minutes. Stir in the tomatoes and serve.