Preheat oven to 400°F.
Preheat a cast-iron skillet over medium-high heat. Add half a turn of the pan of oil and the bacon; render then remove to plate. Add leeks to bacon drippings and wilt; add apples and toss to coat. Season with salt, pepper, nutmeg, sage and light brown sugar; cook to soften, 7-8 minutes, turning frequently. Add lemon zest and juice then remove apples to a serving bowl, add bacon and toss; cover with foil to keep warm.