Step 1 Preheat oven to 400°F (205°C).
Step 2 Cook potatoes the first time: Gently scrub potatoes but do not peel. Pierce all over with a fork so that steam escapes. Bake 1 hour to 1 hour 15 minutes or until potatoes are tender when pierced. Leave oven on.
Step 3 Alternatively, microwave fork-pierced potatoes for 10 minutes, turning them halfway through, or boil for 15 minutes.
Step 4 While potatoes cook, prepare your filling: Tear kale, chard, or spinach leaves from stems. Plunge leaves in cold water to remove dirt or grit. Tear leaves into large chunks. Heat a skillet over medium-high, add greens and a pinch of salt. Cook them in the pan with just the water clinging to the leaves until wilted. Transfer to a colander, wring out any extra moisture when cool. Finely chop greens.
Step 5 Trim leek to yellow and pale green part. Halve lengthwise. If gritty, rinse in cold water, then pat dry. Cut leek halves lengthwise again and thinly slice.
Step 6 Heat a large skillet over medium heat; add butter and oil. Once warm, add leek and reduce heat to medium-low. Cook until tender and sweet for about 10 to 15 minutes, occasionally stirring. Avoid browning. Add chopped greens, warm with leeks for 1 minute. Transfer mixture to a bowl.
Step 7 Prepare potatoes: When potatoes are cool, halve lengthwise and scoop out all but the last 1/4-inch thickness of skin and potato, leaving a shell for stability. Arrange potato shells on a baking sheet. Mash potatoes, leeks, and greens until smooth. Stir in sour cream, 3/4 of the cheese, salt, and pepper. Heap filling in prepared potato skins. Sprinkle with remaining 1/4 of cheese.
Step 8 Bake potatoes a second time: Bake for 20 to 30 minutes, until bronzed and crisp on top.
