Preheat oven to 350.
Rub the sweet potato with a little of the oil and place in the oven to roast until very tender, approximately 60-80 minutes depending on size.
Place the rest of the oil in a sauté pan over a medium heat. Add the garlic and cook gently for a minute. Add the black beans and cook for four minutes, adding a little water if the pan gets dry. Add the greens, parsley and walnuts. Cook for five minutes or more until the greens are soft. Add the feta and stir through gently.
Season generously with sea salt and cracked black pepper.
To serve: Slice the sweet potatoes down the middle at the top. Use two spoons to gently pry it apart and fill the cavity with the bean mixture. Drizzle with additional olive oil and a good grind of cracked black pepper.