We’ll admit it: The Metropolitan Museum of Art can be slightly intimidating, considering the institution’s size and the sheer volume of its holdings. This two-hour lesson, put on by Big Onion Walking Tours, is intended to demystify the massive art museum by offering tidbits about its history and collection, as well as its impact on the city. And we’re not kidding about the museum’s rather large collection: At any given time, only about 3 percent of its holdings are on view. Location disclosed with ticket purchase. Advance tickets required.