Heat oil in a 10-quart pressure cooker, over med-high heat. Sear the roast on all sides.
Add all ingredients, except flour (or cornstarch) to pan, arranging potatoes on top of roast.
Close and lock lid. Bring to pressure (according to manufacturer's instructions for your brand) and cook for 45 minutes at high pressure.
Release pressure using natural release method. This may take about 10 minutes.
Remove vegies and roast from pan and keep warm. Mix about 2 tbls flour (or cornstarch) with a little cold water and stir into sauce, cooking until thickened (may take more flour, depending on how thick you want the gravy).