How to make it
Make dressing by combining mustard, garlic, oregano, and salt in a bowl. Whisk lemon juice. Slowly whisk in oil.
Make herb brush by bundling together rosemary, oregano, and thyme at their stems. Make a handle out of the base of a whole leek by removing the green tops and half an inch from white bottom. Make a lengthwise cut so that you can loosen and remove the center of the leek. Place the stems of the herb bundle inside of the leek handle and tie with twine.
Prepare grill for medium-high heat. Rub sweet potato with dressing and secure it on a rotisserie spit. Roast until cooked fully through but not falling apart, 45 minutes to an hour, using the herb brush to occasionally base with the dressing.
Remove sweet potato and allow it to cool. Slice into thick rounds and serve over a salad of kale, romaine, walnuts, and olives tossed with more dressing.