Put the onion, carrots, celery, potatoes, corn, and bay leaf in a 4-quart slow cooker. Sprinkle with 1 teaspoon salt and 1 teaspoon pepper, then pour in 2½ cups water. Cook, covered, on the low setting for 4 to 5 hours, until the vegetables are soft.
Ladle some of the liquid into a small cup and add the flour to the cup, stirring until smooth. Pour the slurry back into the soup and stir to combine; cook on the high setting for 30 minutes, or until thickened slightly.
Mince the celery leaves. Stir in the cream, along with the parsley and celery leaves. Taste and add more salt and pepper if necessary. Serve hot.
If you like, begin the chowder by cooking 8 slices bacon in a large deep skillet or pot over medium heat until crisp. Remove the bacon to paper towels to drain; crumble and set aside. Pour off all but 1 tablespoon of the bacon fat and return the skillet to medium-high heat. Add the onion, carrots, and celery, then sprinkle the vegetables with the flour. Cook, stirring, until the flour is beginning to turn golden brown. Add 2½ cups water and stir to loosen any browned bits on the bottom of the pan; pour the vegetables and liquid into the slow cooker. Add the potatoes, corn, bay leaf, and salt and pepper to taste and cook on the low setting for 4 to 5 hours. Stir in the cream, parsley, and celery leaves, season again, and serve sprinkled with the bacon.