First, there's the "What is it?" Jewish brisket is not corned beef. Nor is it Texas-style barbecued brisket or French pot au feu, although the cut is the same. One of the nine beef primal cuts, it's from the pectorals, and because cows don't have collarbones, this big muscle supports a lot of bovine body weight. (Read: Cook this cut low and slow to maximize tenderness.)
Then there's the "Why is it the most traditional Passover protein?"
Ashkenazi Jews dust off their brisket recipes from Rosh Hashana to Passover, which begins Friday, most folks claiming they have the ultimate secret to a tender and sumptuous dish. "A can of Coke is the magic!" "Onions, no potatoes!" "Chili sauce and a packet of onion soup mix!" "Brown the meat first!" No matter the minutiae, it is tradition to braise this kosher cut, and essential that you buy a brisket with decent marbling and a nice fat cap that will melt into your sauce and provide richness to a cut that can be a little stringy. As with most traditions, it's because it's how Grandma did it, but also because it's convenient for Passover hosts, best made beforehand, even the day before.
The key to good brisket is cooking it slowly, immersed in liquid in a tightly covered pan (a slow cooker is perfect), and then slicing it thin against the grain. You can slice it, cool it to room temperature and then reheat it slowly before serving, arraying it on a platter with its sauce. This recipe comes from my mother-in-law's sister.
Laura Reiley, Times food critic