I used flour to thicken the sauce, but you can omit it and opt for another method. Once the pot roast is fully cooked, transfer it to a plate along with the vegetables. The sauce will be left in the pan and can either be heated over medium-high heat to thicken, or you can use an immersion blender or ladle into a countertop blender and puree a few potatoes along with the sauce to thicken it. You can also make a slurry using 1-2 tablespoons of cornstarch and a few tablespoons of water or 2-3 tablespoons of flour and water, mix together, and add to the simmering sauce. The cornstarch mix will thicken it more quickly (1 minute) than the flour slurry (~3 minutes) and cornstarch has about double the thickening power of flour.