Add onion, celery, carrot, garlic, and stems from Swiss chard.
Cover and cook until softened, about 8-10 minutes, stirring occasionally.
Add cooked vegetable mixture, lentils, potatoes, broth, and soy sauce in a 4- to 6-quart slow cooker.
Stir to combine, cover, and cook on low heat for 8 hours.
Just before soup is finished cooking, bring a large pot of water to a boil. Place reserved chard leaves in boiling water and cook until tender, about 5 minutes.
Drain leaves well and stir into soup. Season with salt and pepper to taste.
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