There are many ways you can thicken a stew. A few among them are - 1) The addition of potatoes helps to thicken the stew. 2) The addition of cornflour thickens it. You can add 1 tbsp cornflour for 1 cup of liquid in the stew. 3) You can also add some all-purpose flour while sautéing the vegetables. Cook the flour well (until brown) and then add broth. The flour will thicken the stew.