DO NOT USE A GLASS PAN FOR THIS RECIPE.
Pre-Heat the oven to 500 degrees.
Prep the roast by making 4-5 slits in the roast and filling the holes with salt, pepper and 1/2 of a clove of Garlic.
Now Salt and Pepper the rest of the roast.
Put the Roast in the oven and sear for 20 minutes.
Then add the water, the chopped onion and reduce the temperature to 275 degrees.
Cook for 20 minutes per lb.
I like mine rare so that is what you will get if you cook for this period of time.