Poke holes in sausages and put in a large nonstick skillet. Add 1/4 cup water; bring to a boil. Lower heat and cook sausages, turning, until lightly browned and water has evaporated, about 10 minutes. Transfer to a plate. Heat oil in skillet over medium-high heat. Add onion and garlic; cook, stirring often, about 3 minutes. Add beans, tomatoes and thyme.
Put half of bean mixture into a slow cooker. Arrange sausages on top, followed by remaining beans. Cook on high for 4 hours. Check after 2 hours and add water if mixture looks dry.
Remove sausages; slice thickly. Season beans with salt and pepper. Return sausage to stew. (Stew can be kept, frozen and tightly sealed, for up to 1 month.)