There are a few ways to thicken it: 1. Dust the beef pieces with flour and brown them on all sides prior to adding them to the slow cooker. The flour will help to thicken the stew. 2. Add potatoes. The starch in the potatoes will act as a thickener. 3. My chosen method is to add a cornstarch slurry (cornstarch mixed with cold water) to the dish after it's done cooking, then continue to cook it until the cooking liquids thicken.