Season roast with salt and pepper. Place roast in the inner pot. Arrange onions on top of roast.
In a medium bowl, combine celery flakes, garlic powder, broth, and Worcestershire sauce. Pour over roast. Add bay leaves to the pot. Arrange potatoes and carrots around roast.
Close and lock the lid and turn the steam release handle to Sealing. Set your Instant Pot to pressure cook on High for 45 minutes.
When the cooking time is done, press Cancel and let stand, covered, for 10 minutes, then turn the steam release handle to Venting. When the float valve drops down, remove the lid. The roast should be fork-tender. (If more cooking time is needed, continue pressure cooking on High for 5 minutes, then quickly release the pressure.)
Using a slotted spoon, transfer potatoes and carrots to a bowl. Using tongs, transfer roast to a cutting board. Slice roast across the grain and transfer slices to a serving platter.
Skim off fat from cooking liquid and drizzle roast with some of the liquid. Arrange potatoes and carrots around roast.