Make the potatoes:Place potatoes in a pot. Add the salt, herbs and garlic (if you are using). Cover with approximately one inch of cold water. Bring the pot of water to a boil, then turn off the heat. Let the potatoes cool completely in their liquid before proceeding. (See notes above re timing.)Make the harissa:Place the garlic, herbs, chili, lemon or vinegar, cumin (if using), and salt into a food processor and pulse a few times to combine. Then, while the food processor is running, drizzle in the olive oil until everything is combined. (I actually just throw everything in together and puree it all at once.) Taste. Adjust seasoning as necessary. You may need more vinegar for bite. You may need more salt. You might want to thin it out with a little more olive oil. Set aside.When the potatoes have cooled completely, halve them and place in a mixing bowl. Toss with dressing. Season with more salt if necessary. Note: The sauce loses its bright green hue shortly after it is tossed with the warm potatoes, but it is no less delicious.