Set the Instant Pot to Normal/Medium and Press Start.
Once hot, add olive oil and onion to the inner pot and cook for 3 Minutes.
Add the ground beef and italian sausage and season with salt and pepper then let sear until browned on the bottom, about 2-3 Minutes.
Break up beef, then add garlic and sauté it until you smell the garlic fragrant.
Once the beef is cooked, add rice, water, diced tomatoes, Italian herbs, marinara sauce, Shishito diced pepper, Worcestershire sauce, and season with salt and pepper to taste. Stir it for 2-3 Minutes then press the Cancel button.
Transfer content of inner pot to large mixing bowl and set aside. Clean inner pot and place in the trivet along with water.
Now prepare the bell peppers. Remove and discard the tops, seeds, and membranes of bell peppers. Place the bell peppers hollow side up along the trivet.
Scoop mixture from large mixing bowl into the individual bell peppers.
Pressure Cook at High pressure for 7 Minutes. When the time is up, allow for 5 Minutes Natural Release, then quick release the rest of the pressure.
Remove the bell peppers from the trivet. Serve while hot. Garnish with feta cheese and fresh parsley if desired!