Reduce the heat to medium and add the shallots and onions to the pan. Cook, stirring, until the shallots are soft. Add the tomato paste and flour and cook, stirring, for another minute. Add the vinegar, bring to a simmer, then stir in the chicken stock. Bring to a simmer, making sure to scrape the bottom of the pan. Return the chicken to the pan with half of the tomatoes and the tarragon. Cover and simmer for 10 minutes, stirring occasionally. Add the remaining tomatoes, cover, and simmer for another 20 minutes, or until the chicken is cooked through.