Pearl onions are small, round onions that have a mild flavor and sweet taste. They are usually about the size of a large marble and can range in color from white to yellow to red. Pearl onions are often used in recipes such as beef stroganoff, braised dishes, salads, and even as appetizers. When cooked, pearl onions become tender and slightly sweet making them an ideal addition to any dish. The grocery store has a few places that they tend to store these little things. Some stores sell fresh pearl onions and others only have them in the frozen section. If you can only find frozen, just know they are already peeled. I would also not add the frozen ones in when adding the mushrooms to the pot or they’ll fall apart. I’d add them in when I add the stock and wine to the pot (Step 8). If you can't find pearl onions, you can use shallots as a substitute.