Turn Instant Pot to the Saute mode and add the olive oil.
Once heated, add the onions, mushrooms, and roasted red peppers. Cook for 3 minutes, stirring frequently, until the onions are translucent.
Add the minced garlic, stir, and cook for 30 seconds.
Next, add the ground beef, Italian seasoning, basil, salt and pepper (as listed or to taste). Using a wooden spoon, break up the ground beef as it cooks, stirring very well.
Once the beef has browned and is no longer pink, cancel the saute setting on the Instant Pot.
Break the spaghetti noodles in half and add them in layers to the pot, fanning them out. You want to make sure they aren't laying together in a heap, otherwise they will clump together while they cook.
Cover the noodles with chicken stock, marinara, and the chopped tomatoes. Stir gently to ensure that all of the noodles are covered.
Place the lid on the pot and turn the valve to 'sealing.'
Cook on HIGH PRESSURE for 5 minutes and allow a 5 minute natural release followed by a quick release.
Open the lid and give the spaghetti a gentle stir. There may still be some liquid present. Just let it sit for 3-4 minutes. The pasta will absorb the liquid.
Serve garnished with parmesan cheese and freshly chopped parsley.