Mustard greens have a strong, peppery, almost bitter flavor. Kale, spinach, or Swiss chard can be substituted for a more mellow taste.
Heat an 8-quart soup pot over medium heat. Add bacon; cook, stirring occasionally, until crisp. Drain on paper towels.
Add shallots and garlic to pan with bacon fat. Saute until tender, about 2 minutes.
Add greens and stock; toss well to coat. If all greens don't fit, let wilt in pan until there is room to add more. Cover; cook until all greens are wilted, about 10 minutes, tossing occasionally. Uncover; cook until greens are tender and most liquid has evaporated, about 10 minutes more.
Stir in the vinegar, season with salt and pepper, and serve warm garnished with the bacon and hot sauce, if using.