Turn your Instant Pot to SAUTE or Ninja Foodi to SEAR/SAUTE. Press START to begin heating the pressure cooker.
Add the olive oil to the pot. Add the bacon slices and sauté for 2 minutes. Add the mushrooms and sauté with the bacon for 1 more minute. Add the garlic, shallots, and rosemary and sauté for 1 more minute.
STOP the SAUTE function and transfer the filling mixture onto a plate; set aside.
Pour some of the water into the bottom of the pot and use a wooden spoon to deglaze the pot. Be sure to scrape up any brown bits that might have stuck to the pot. Pour the remaining water into the pot.
Butterfly and stuff the pork
Using a sharp large knife, slice the pork horizontally along the long side, cutting almost to the center of the meat, and then carefully continue cutting to the other side, unrolling the pork as you cut (watch the video to see how to butterfly a pork loin).
Lay the butterflied pork loin flat on the cutting board and if it is thicker than ½ inch, cover with plastic wrap and pound it thinner with a mallet, rolling pin, or potato masher until the meat is about ½-inch thick. Remove the plastic wrap.
Spread the mustard across the pork. Then sprinkle the salt and pepper evenly across the pork. Spoon the bacon mushroom filling mixture evenly across the pork.
Working from one longest side to the other, very carefully roll the pork up to the end. Secure the seams with toothpicks or use kitchen string to tie and hold the rolled and stuffed pork loin together.
Place a rack into the pot and spray with cooking spray. Place the stuffed pork loin onto the rack.
Assemble the pressure lid, making sure the pressure release valve is in the SEAL position. Select PRESSURE (or MANUAL on some Instant Pots) and set it to HI. Set the time for 20 minutes, and start.
When the cooking is complete, allow the pressure to release naturally for 10 minutes. After 10 minutes, quick release the remaining pressure by moving the pressure release valve to the VENT position. Carefully remove the lid when the unit has finished releasing all pressure and the knob is all the way down.
Cooking is complete when the internal temperature of the meat reads at least 145°F on a food thermometer. Let rest for 10-15 minutes before removing the toothpicks and/or kitchen string and slicing.
Make the gravy
Remove the pork loin and the rack from the Instant Pot. Turn the Instant Pot to SAUTE in HI. Add the ¼ cup flour to the pan drippings inside the Instant Pot and whisk until smooth and thick. Optional: add some chicken stock if your gravy is too thick and needs to be thinned out. Season with black pepper if desired.
Notes
Gravy: if the gravy is too thick, you can thin it out by whisking in some chicken stock until it reaches your desired level of consistency. If your gravy is too thin, gradually whisk in more flour until it reaches your preferred level of thickness.