In a large Dutch oven or heavy pot, cook sausage over medium-high heat until the meat crumbles and begins to brown, about 5 minutes. Add onion, celery and garlic and cook until softened, about 5 minutes.
Add stock, lentils, red pepper flakes and tomatoes and bring to a boil. Reduce to a rapid simmer, partially cover, and cook until lentils are tender, 30-35 minutes.
Add kale and simmer, uncovered, until tender, 2-3 minutes, Stir in vinegar and season with salt and pepper, to taste. Sprinkle with chopped parsley before serving.
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Notes
To Store: Store in a tightly covered container in the fridge for up to 5 days.
To Reheat: Gently reheat on the stove over medium-low heat until hot. You can also microwave the leftovers.
To Freeze: Store in a tightly sealed container in the freezer for up to 3 months. Allow to thaw overnight in the refrigerator before reheating.