In a medium pot over medium heat, add the oil. When it is hot, add the onion and cook until it begins to soften, 2 to 3 minutes. Add the cabbage, season with salt and pepper, and cook until the cabbage begins to soften, about 3 to 4 minutes. Add the chicken stock and cider, bring it to a boil, reduce to a simmer, and cook until the cabbage and onions are soft, about 8 to 10 minutes. Add the dumpling and heat through. Taste and adjust seasoning with salt and pepper.