"I know it sounds weird. . . apples and lentils? But it's a delicious, light, and healthy soup. Serve this with some homemade bread, a salad, and it's a meal. This is a regular at our family table; I hope you like it."
Heat olive oil in a large pot over medium heat until hot; stir in onion and apples and cook and stir for 4 minutes. Stir flour into apple mixture; cook and stir for 2 minutes.
Whisk chicken stock into apple mixture; bring soup to a boil. Stir lentils and celery seed into soup; return mixture to a boil, reduce heat to low, and simmer until apples are tender, about 45 minutes.