Heat the Instant Pot on High Sauté. Then add the onions, celery, and carrots. Then stir in the salt and pepper. Sauté the veggies, stirring them frequently with the lid off for 5 minutes or until the veggies start to soften.
Add the garlic and the Herbs de Province. Stir the ingredients for about 30 seconds until the veggies are covered with the herbs.
Turn the Instant Pot off. Immediately add the sherry to deglaze the Instant Pot. Use a wooden spoon or soft utensil to clear the bottom.
Add the mustard, lentils, rice, and vegetable broth. Give the bottom of the Instant Pot another stir to ensure there is no food stuck to the bottom.
Secure the Instant Pot lid and move the steam release handle to the Sealing position. Set the cook mode to High Pressure for 16 minutes and turn off the warming button.
When the pressure-cooking process ends, leave the Instant Pot alone. Allow the steam to naturally release for another 16 minutes.
Move the steam release valve back to unsealing and carefully remove the Instant Pot lid. Give the ingredients a stir and fluff up the rice. Taste and adjust the mustard or Herbs de Province. You can also add more salt or freshly ground pepper if desired.
Serve individually or in a big bowl. Garnish with chopped parsley if desired.