Red onions have the best flavor for this soup, but yellow onions will also work.
Place onions in soup kettle and cover with cold water. Bring to a boil, reduce heat, and simmer, partially covered, 10 minutes. Drain. Melt butter in heavy skillet and saute onions 10 minutes. Sprinkle with sugar and simmer, partially covered, 45 minutes. Press onions with chicken stock through food mill and return to kettle. Add cinnamon and lemon juice. Season with salt and pepper. Heat thoroughly. Garnish with a sprinkling of cinnamon.