Place chicken in water and season with salt and pepper. Bring to a boil. Skim fat that rises to top with large spoon. Repeat 2 more times or as needed. Add vegetables and herbs. Cover and let simmer for 3 hours.
Place chicken skin and onion in a saute pan over low heat. Render fat out of chicken skin. Strain and reserve fat. Discard solids. (Fat can be refrigerated up to 1 week). Lightly beat eggs then add chicken fat. Stir in matzo meal. Season with salt and pepper and mix well. Add seltzer and chicken broth. Stir in, but don't overwork batter, as that will cause tough and chewy matzo balls. Cover mixture and refrigerate 30 minutes to 2 hours. Form into 2-inch balls. Drop softly with slotted spoon into chicken soup or salted water. Bring to boil. Cook 30 to 40 minutes, covered tightly (no peeking.) Serve in strained chicken soup or broth.