How to Make It
Heat oil in a large skillet over medium-high. Add onion, sausage, thyme, and garlic. Cook until sausage is browned and onion is tender, stirring occasionally to break up sausage, 8 to 10 minutes. Transfer to a 6-quart slow cooker, and stir in stock and peas. Cover and cook until peas are tender, about 4 hours on HIGH or about 8 hours on LOW.
Stir in kale, tomatoes, and salt. Cover and cook until kale is wilted, about 10 minutes more. Stir in vinegar and pepper.