Bring 2 quarts of water to a boil and add 1 tablespoon salt. Add the cabbage and blanch for 1 minute. Drain the cabbage in a colander, refresh under cold running water until cool, and reserve.
Put the bacon in a large skillet and cook over medium heat until the fat is rendered, 4 to 5 minutes. Pour off all but 1 tablespoon of the fat. Add the onion and sage and cook, stirring, until the onion is wilted but not browned, about 10 minutes. Add the blanched cabbage and cook for 2 more minutes. Stir in the flour and cook, stirring, 3 minutes. Pour in the stock, bring to a boil, reduce the heat, and cook until thickened, 2 to 3 more minutes. Season with 1/2 teaspoon salt and the pepper and serve.