Trim fat from roast. Coat a 4-quart pressure cooker with cooking spray; place over medium-high heat until hot. Add roast; cook until browned on all sides. Add onion and next 6 ingredients. Cover cooker with lid, and seal securely. Place pressure control over vent and tube. Cook over high heat until pressure control rocks back and forth quickly. Reduce heat until pressure control rocks occasionally; cook 40 minutes.
Run cold water over pressure cooker to reduce pressure rapidly. Carefully remove lid so that steam escapes away from you. Transfer roast to a serving platter; set aside, and keep warm.
Skim fat from pan juices. Combine flour and water, stirring until smooth; add flour mixture to juices. Cook over medium heat, stirring constantly, until thickened. Serve gravy with roast.