In this one-pot dish, smoky bacon, earthy lentils, and caramelized vegetables get a kick from ginger, tart apple, and tangy buttermilk. It's great for satisfying a crowd, or it will freeze well for healthy comfort in a hurry.
Slice bacon into 1-inch pieces. Heat vegetable oil over medium heat in a 4-quart Dutch oven or saucepan. Add the bacon and cook till crispy, 8-10 minutes. Meanwhile, cut the leek in half reserving the light green part (discard or use the dark upper stem for another use such as stock). Slice lengthwise, then thinly slice into thin half-moon rounds. Set aside. Peel and slice the carrots. Cut into quarters lengthwise, then dice. Set aside. Trim celery then cut in quarter strips, lengthwise and dice.
Drain the bacon on paper towels and pour off all but 2 tablespoons bacon fat (save the bacon fat by pouring into a room-temp glass and keep in the fridge). Add leeks, carrots, celery, and a pinch of salt to the pan. Stir then sweat over low heat until vegetables are tender, 5-7 minutes. Meanwhile, use a knife to remove the ginger peel. Cut ginger in half, then thinly slice into sheets, then matchsticks and finely chop. Add to the pot. Peel and dice garlic and add to the pot.
When the vegetables are soft and lightly caramelized, add cumin. Toast in the skillet for 30 seconds. Add lentils, black pepper, chicken stock, and apple cider. Cover, turn up heat to medium and bring to a heavy simmer, 5 minutes. Reduce to a low simmer and cook 30 minutes.
While the soup is simmering chop the apples by slicing around the core in thin rounds, then matchsticks and finally a small dice. Remove rosemary needles from stem by holding the sprig at the top, then sliding your fingers down to the bottom stripping off the needles. Bunch needles together and mince. When the soup is “soupy,” stir in the apples and rosemary and cook 2-3 additional minutes.
Taste soup and add salt if necessary. Serve in individual bowls. Garnish with a drizzle of buttermilk and a sprinkle of bacon.