Sprinkle the beef all over liberally with salt and pepper.
Select SAUTE and adjust to More on the Instant Pot and add the oil.
Once the oil is hot and begins to shimmer, add the beef. (You will probably need to do this in 2 batches, as you don't want to overcrowd the beef as it browns).
Brown the beef for 3 minutes, then turn the pieces over, and brown for another 3 minutes.
Transfer the beef to a plate.
Leaving the IP on high heat, add the mushroom and cook, stirring occasionally, until the are starting to turn golden brown, about 4 - 5 minutes. Transfer to a bowl and set aside.
Press CANCEL and then add back into the pot the seared beef, as well as the beer, stock, onion quarters, garlic, Worcestershire sauce, mustard, thyme, bay leaf and salt and mix together.
Secure the lid and push the Pressure Release to Sealing. Select the MEAT/STEW or PRESSURE COOK setting and set the cooking time for 30 minutes at high pressure.
Carefully perform a quick release by moving the Pressure Release to Venting.
Once the steam is fully released, open the pot and remove the onion quarters, bay leaf and thyme sprig.
Stir in the carrots, potatoes and pearl onions.
Secure the lid once again and push the Pressure Release to Sealing. Select the PRESSURE COOK or MANUAL setting and set the cooking time for 4 minutes at high pressure.
Perform a quick release by carefully pushing the Pressure Release to Venting.
In a smal bowl, mix together the corn starch and the water.
Open the pot and stir in the reserved mushrooms and the cornstarch mixture. Stir occasionally for about 3 - 5 minutes until slightly thickened.
Taste and add a little more salt, if desired.
Ladle into bowls and garnish with thyme leaves, if desired.