Preheat the oven to 400°F. Line two large baking sheets with parchment paper.
Wash the potatoes really well and dry them. Cut them into ½" chunks.
Place the potatoes in a large bowl. Add the olive oil, salt, smoked paprika, cumin, and black pepper. Toss well until all the potatoes are well coated with the oil and the spices.
Transfer the potatoes to the lined baking sheet, making sure you also pour on the oil and liquid remaining at the bottom of the bowl. Bake for 30 minutes or until the potatoes are tender.
To make the Harissa sauce, combine all the ingredients, except for the water, in a medium bowl. Whisk in the water, one tablespoon at a time, until well incorporated (the sauce should have the consistency of a thin ranch dressing).
To serve, pour the sauce over warm potatoes or serve the sauce on the side.