Preheat the oven to 425°F. Make several small slits in the potatoes with the tip of a sharp knife to vent them.

Bake the potatoes for 1 hour, or until they can be easily pierced with a knife. Remove from the oven, reduce the temperature to 350°F, and allow the potatoes to cool for a few minutes before handling.

While the potatoes bake, heat 2 tablespoons of the butter in a large pan over medium heat. Add the onion and sauté until soft and translucent, 2 to 3 minutes. Add the garlic and continue cooking 1 minute longer. Add the sun-dried tomatoes and pine nuts, sauté 1 minute, then add the spinach, and continue cooking until wilted and nicely combined with the other ingredients. Season to taste with salt and pepper and set aside.

Once the potatoes have cooled, cut a 1/4-inch thick slice from the top of each. Using a teaspoon, scoop the flesh into a mixing bowl, leaving about 1/4-inch of the potato "wall" in tact. Be careful not to break the skins.

Add the remaining 4 tablespoons of butter along with the cream (if using) to the potatoes and mash with a fork. Fold in the goat cheese, reserving a little for garnish.

Spoon equal portions of the potato-cheese mixture back into each potato, pressing it against the sides to make a cavity for the spinach down the center. Add a portion of spinach to each potato, and top with a little of the reserved goat cheese.

Return the potatoes to the oven and bake for 5 to 7 minutes, or until the cheese on top has softened.