When we call for a “casserole” we mean a shallow glass or ceramic dish. Sometimes these sorts of dishes have lids, but they are not necessary; you can use foil to cover dishes that need to be cooked covered. If you’re only going to get one casserole dish, go for a 9-by-13-inch (or similar-size) one that holds 3 quarts. You may also want a smaller 7-by-11-inch or 2-quart dish. Glass dishes are great because they’re inexpensive and available at the supermarket. Spend a little more and you can get a ceramic dish that looks pretty on the table and is safe to use under the broiler.