Pierce the potatoes and microwave for 20 minutes or until tender when gently squeezed. Let cool slightly.
Meanwhile, melt the butter in a medium skillet. Add the leeks and cook until very soft, about 10 minutes, stirring frequently. Add the spinach and cook for a few minutes more until the spinach is cooked and any liquid has been absorbed.
Preheat oven to 400° F.
Cut the potatoes in half and scoop out the insides, leaving a rim of potatoes in the skins. Set skins aside.
Mash the potatoes with the milk until smooth, then stir in the leeks, half the cheese, salt and pepper. Lightly stir in the roasted peppers and basil.
Place the potato skins onto a baking sheet and fill with the potato mixture; sprinkle with remaining cheese. Bake for 15 minutes.