In a large heavy pot heat olive oil over moderate heat and cook the onions until translucent. Add the minced garlic and crushed red pepper flakes and cook, stirring, for 1 minute. Stir in the balsamic vinegar and the brown sugar and bring to a simmer. Add the cabbage and stir to combine. Add the tomatoes and the chicken stock and bring to a simmer. Season with salt and pepper, to taste. Simmer the soup, uncovered, for 30 to 40 minutes until the cabbage is very tender. Taste soup and adjust seasoning with more balsamic vinegar, brown sugar, salt and/or pepper. Stir in parsley. Serve hot with a dollop of sour cream and chives.