Rinse roast and pat dry. Rub on all sides with salt and pepper and set aside.
Melt the butter in the pressure cooker. Place the roast and onion in the cooker and brown over medium high heat, turning occasionally, until nicely browned on all sides. Add herb sprig and water.
Lock lid into place and bring to 15 lbs pressure. Adjust heat to maintain pressure and cook for 50-60 minutes.
Allow pressure to drop by the naturally and remove the lid. Set roast and onion quarters aside.
Dissolve corn starch in 1/2 cup hot water. Bring juices to a boil in pressure cooker. Slowly add cornstarch and and water, whisking or beating with an immersion blender all the while. Reduce heat and let gravy simmer, whisking or beating all the while, until it reaches desired thickness.