Set your Instant Pot to the sauté setting. Cut your pot roast into large chunks, this will make it easier to sear the outside. Add the olive oil to the pot and sear each side of the beef, remove and set aside.
Add in the onions, salt and pepper, then sauté for about 2 minutes, then add in the garlic and continue sautéing for another 20 seconds. Add in the white wine to deglaze the pot and scrape up the brown bits of flavor.
Allow to cook for another minute, then stir in the chicken stock, tomatoes, tomato paste and Herbs de Provence. Add the beef back in and lock the lid in place, set the valve to sealing.. Turn the pressure cook/manual setting to high pressure for a time of 20 minutes per pound of beef. After time is up, let it naturally release for 20 minutes before switching to manual release.
Remove the meat and set aside to pull apart. You will not need a knife, it will fall apart easily! Add in the mascarpone cheese and Xanthan Gum and mix using an immersion blender. If you don't have an immersion blender, you can mix in a food processor or Vitamix blender.
The white wine is optional, it gives the pot roast and gravy a great depth of flavor. You can certainly omit it, just replace with more chicken stock.