Preheat oven to 300 degrees F. and get out a roasting pan that the oblong-shaped brisket will lie in snugly.
Slice the onions, stripping away the skin that unwraps itself and arrange them largely in the center of the pan, to create an onion platform for the brisket to lie on.
Place the brisket on top of the bed of onions, fat-topped side uppermost.
Whisk together the remaining ingredients and pour them over the brisket, then cover the pan with aluminum foil, sealing it tightly - make a double layer if you want - and place in the low oven for 3 1/2 hours.
Remove from the oven and transfer the meat to a board to rest a bit.
Transfer the onions to a blender with a ladleful or two of the liquid from the pan and blend until smooth, then add back to the rest of the liquid in the pan and stir to mix.
Slice the brisket in diagonals across the grain and then, if you want, cut the slices in half across before popping them into the onion-thickened gravy in the pan. You can even serve this straight from the pan!