Remove kernels from cob, keeping both. In a stock pot, heat olive oil over medium-low heat. Add diced onion and cook until translucent, 6-7 minutes. Stir in sweet corn and cook for another 5 minutes, letting corn heat up and soften slightly. Pour in vegetable broth, mix in the seasoning, and add the corn cobs. Bring to a boil, reduce to a simmer and cook for 15-20 minutes while making the harissa.
After the time, remove the cobs and puree the soup in a blender. If smooth soup is desired, pass liquid through a fine mesh sieve back into the pot, pressing down with the back of a spatula to remove all liquid. If not, return all soup back to pot. Stir in heavy cream and heat soup over low until hot, but not boiling.