When it comes to holiday entertaining, shellfish rule the day. Somehow these lowly mollusks have taken on dinner party stature, despite being relatively inexpensive, and even if you live miles and miles away from the nearest source. Frozen cockles are easy to find at most supermarkets, and their taste and texture should be fully intact when properly thawed and cooked, here with crushed garlic cloves, fresh lemon juice, and ample cilantro.