Which old Beijing instant-boiled mutton is the most authentic?

I think Nanmen instant-boiled mutton is the most authentic instant-boiled mutton in old Beijing.

Since the establishment of 1994, Nanmen Shuan rou has won high recognition from Beijing and the public for its honest management and unique formula. One of the characteristics of South Gate hotpot is fresh-cut mutton tendon, and poplar and melon strips are also a highly respected dish.

The traditional Muslim hot pot in Beijing was called "Hongyuan Hot Pot" at the beginning of its opening. Because the head office is located in the south gate of Tiantan Park in the South Second Ring Road, it is often affectionately called "South Gate Hot Pot" by customers and friends.