Complex sentence: Sinigang is often a stewed fish, pork, chicken (usually called "sinampalukan" and cooked with tamarind essence), shrimp, and beef, but the key to a good sinigang is the ingredient which makes it sour.
Split into simpler sentences: Sinigang is often stewed fish, pork, chicken (usually called "sinampalukan" and cooked with tamarind essence), shrimp, or beef. But the key to a good sinigang is the ingredient is not the meat one uses but the agent which makes it sour.