Preheat oven to 200 °C. Brush the yam roots or sweet potatoes with oil all over. Bake for about 45 min until they pierce slightly.
Meanwhile, in a baking bowl, combine the butter, chopped scallions, berries, brown sugar, crem de cassis, salt and pepper. Knead well with a spoon or electric hand mixer.
When yams or possibly sweet potatoes are cooked, remove from heat and divide. On each half put a good unit of the onion berry butter mold. Bring to table hot.
Garnish the plate with whole berries.