1. Preheat oven to 350°F. Combine salt, pepper, paprika, and oregano in a small bowl, then rub all over brisket.
2. Heat oil in an ovenproof enameled cast-iron pot or other heavy pot
with a tight-fitting lid, just large enough to hold brisket snugly, over
medium-high heat. Add brisket to pot and brown on both sides, about 10
minutes per side. Transfer brisket to a platter and pour off fat from
pot. Add stock, tomatoes, and bay leaves to pot and scrape any browned
bits stuck to bottom of pot with a wooden spoon. Return brisket and any
accumulated juices to pot and scatter onions and garlic over meat. Cover
pot, transfer to oven, and braise brisket for 1 hour. Uncover pot and
continue to braise brisket for 1 hour more.
3. Push some of the onions and garlic into braising liquid surrounding
brisket. Check meat for tenderness by piercing center with tip of a
sharp knife; it should slide in easily when fully cooked. Cover pot,
return to oven, and continue to braise brisket, checking on tenderness
of meat occasionally, until very tender when pierced, up to 2 hours
more.
Transfer brisket to a cutting board and loosely cover with foil. The
onions and garlic in the pot should be very soft, and braising juices
should be rich and saucy. If juices are thin, transfer pot to top of
stove and simmer over medium heat until juices thicken, about 5 minutes.
Slice brisket across the grain and transfer to a warm serving platter.
Spoon onions, garlic, tomatoes, and juices on top. Discard bay leaves
before serving.