Use two long sheets of aluminum foil to place them in the bottom of a roasting pan side by side but overlapping. Make sure the roasting pan fits the brisket nicely, not too bit and not too small.
Take brisket flat out of the package, lay it in the roasting pan fat cap up and pat it dry with a paper towel.
Season it with salt all over and pat it in evenly. Wrap brisket in a couple of layers of foil, covering it tight.
Refrigerate the brisket overnight or up to 12 hours. Take the brisket out of the refrigerator an hour before cooking it, unwrap and let it get to room temperature.
Slice all onions thinly, peel and chop garlic, shave and mince ginger, and measure the rest of ingredients.
Preheat a Dutch oven or another pot over medium heat and add some oil. Add onions and cook until completely softened.
Make a little well in the middle of onions and add garlic to the center. Let is saute until fragrant and mix it in.
Sprinkle flour all over the onions and stir to coat them completely.
Start pouring in balsamic vinegar and red wine vinegar while stirring slowly but constantly. Pour in beef stock while still stirring.
Add bay leaves, sugar, garlic powder, onion powder, ginger, salt, and pepper. Keep stirring slowly and the sauce comes to simmer and thickens.
Let the sauce cook for a few minutes.
Carefully transfer the sauce into a food processor or a blender and blend until smooth. (Depending on a food processor or a blender, you may need to wait until the sauce is cooled a little if it doesn't have a vapor cover or a flap for the steam to escape.)
Making the brisket:
Preheat the oven to 300°.
Spread some sauce under the brisket and pour the rest of the sauce over it. Close the foil around the brisket to create a pocket. If the foil sheets don't cover it completely, you may need to cover with another sheet of foil on top and tuck it under.
Place in the oven and cook for about 6 hours. (Estimate your cooking time based on the weight of your brisket. It will take about an hour per pound.)
Open the foil and cook for another 45-60 minutes uncovered.
Take the roasting pan out of the oven and take the brisket out of the pan and onto a cutting board. NOTE: be careful, the brisket will be tender. So use large spatulas or tongs as needed.Loosely tent it with a sheet of foil and let the brisket rest for about 15-20 minutes before slicing.
While brisket is resting, finish the sauce.
Pour all of the leftover sauce from the roasting pan into a sauce pot. Bring it to simmer over medium-low heat and simmer for about ten minutes.
Use a sharp knife to slice across the grain when ready.