Preheat oven to 425°F.

Combine salt, pepper, garlic powder, onion flakes and paprika in a small bowl.

Rub spice mixture over brisket.

Let brisket sit for 30-40 minutes at room temperature.

In a small bowl, combine both mustards, oil and onions. Coat the brisket well.

Bake for 25 minutes in a roasting pan, then reduce heat to 300°F and continue baking for another 3 hours or until brisket is thoroughly cooked.