This makes the best gravy and the most tender roast. My grandmother makes this all the time for company. Everyone always loves it and it makes the house smell wonderful. My favorite part has always been the tender, gravy flavored vegetables.
Warm oil in a pressure cooker over medium high heat.
Add the roast and brown on both sides. Take it off the heat.
Pour Worcestershire sauce over the roast, and place the onion slices on top. Add potatoes and carrots. Pour in enough water to cover 1" above the roast.
Put the lid on and heat on high. When the pressure cooker reaches the proper temperature, the rocker will jiggle audibly.
Adjust the heat to low, or enough to maintain pressure, and keep a slow, steady rocking motion. Cook for 45 minutes to 1 hour. Check on it by lifting the pot and shaking gently to make sure it doesn't cook dry.
Run the lid under cold water to help release the pressure before unsealing.