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Hit "saute" button on Instant Pot. Add sausage and cook until meat crumbles and begins to brown, about 5 minutes. Add onion, celery and garlic and cook until softened, about 5 minutes.
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Add stock, lentils, red pepper flakes, tomatoes and stir.
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Place lid on Instant Pot and make sure the release valve is in the "sealing" position. Press "manual" button and set timer for 12 minutes.
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The Instant Pot will warm up for about 10 minutes and then start cooking. After 12 minutes, quick-release by flipping the release valve to “venting."
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Add kale and vinegar. Stir to combine and season with salt and pepper, to taste. Sprinkle with chopped parsley before serving.