Peel and cut sweet potatoes into 2-inch (5 cm) squares; boil until done, about 15 minutes.
While they are boiling, add the remaining meatball ingredients to a bowl and preheat the oven to 425°F (220°C, or gas mark 7).
Drain the water and with a potato masher, mash the sweet potatoes until they are still slightly lumpy but mostly mashed.
Add the potatoes to the lamb mixture and use your hands to mix well.
On a large baking sheet lined with parchment paper, form little oblong balls–sort of like mini hot dogs.
Bake for about 7 to 10 minutes or until light brown.
Remove from oven and allow to cool.
While the meatballs are cooling, boil a large pot of water. Blanch the Swiss chard leaves for 2 to 3 minutes and remove to place in a bowl of ice water.
To roll, take 1 leaf of Swiss chard and dry it off. Remove the stem at the bottom. Place a meatball in the leaf and roll the bottom part up, then fold over the sides, tucking in as you go, and continue to roll until the meatball is covered with the leaf. Hold in place with a toothpick. Continue until the rest are rolled.
To make the crème, mix together all of the ingredients. The result is a refreshing, aromatic sauce. It’s also a terrific salad dressing, especially over roasted lamb and arugula.