Set a Dutch oven or other large, heavy pot over medium heat. When the pot is warm, add the oil and shallots and cook, stirring, until the shallots are translucent, about 3 minutes. Add the harissa powder and a sprinkle of salt, then stir in the kale and cook, stirring occasionally, until wilted, about 5 minutes. Pour in the stock and bring to a simmer. Add the chickpeas, return to a simmer, and cook for 10 minutes, until heated through and the flavors have blended. Mix in the bread, then taste and adjust with more harissa powder and salt if needed. Portion the stew into four bowls. Flake the tuna and arrange on top of the bowls, dividing it evenly, and then finish with a generous topping of the herbs. Serve immediately.