Hello! With the right recipe, you shouldn't have to precook the potatoes before adding them to the slow cooker. For a good example, click on this recipe: Scalloped Potatoes with Beans. Without knowing the size of your countertop roaster, it's hard to answer the next part of your question. It's probably best to refer to the manufacturer's directions. If you can't find your instruction manual for your Nesco product, you can download one or call to order a new one at the following website: Nesco.