Place sliced onions in bottom of large roasting pan. Lay brisket over onions. Place garlic cloves around brisket.
Sprinkle with steak seasoning, and cover completely with marinara.
Cover tightly and cook for 2.5 hours.
Remove from oven and allow to sit, covered, for half an hour. Transfer brisket to cutting board and cut thinly across the grain.
Place sliced brisket back into pan, arrange carrot chunks and herbs around the meat, and spoon sauce over the top to completely cover. Seal tightly, and cook for another hour.
To serve, discard cooked herbs and replace with several fresh sprigs.
This brisket freezes very nicely – some say it tastes better after being frozen! You can freeze after slicing, and then reheat for 1 ½ hours at 350° to defrost and finish cooking simultaneously.