Prepare your ingredients: dice the onion, slice mushrooms, chop peppers, and shred cheese if needed. Measure your spices and have the marinara ready.

Turn on the Instant Pot to the Sauté setting. Once hot, add the Italian sausage along with the diced onion. Cook until the meat is cooked through (about 5–7 minutes).

Add Italian seasoning, oregano, garlic powder, red pepper flakes, salt, pepper, and the chopped bell pepper and mushrooms (if using). Stir and cook for 1–2 minutes.

Add the broth or water, stirring to deglaze the pot and lift browned bits.

Layer the pasta evenly over the mixture in the pot. Do not stir.

Pour marinara sauce over the pasta, covering it completely. Do not stir.

Cancel the Sauté function, place the lid on the pot, and set the steam release to Sealing. Set Pressure Cook to 4 minutes.

When the cooking is done, perform a Quick Release. Carefully open the lid and stir the pasta.

Stir in half of the mozzarella cheese and some of the pepperoni. Mix in any add-ins.

Sprinkle the remaining cheese on top and arrange the remaining pepperoni slices. Close the lid for a minute or two to let the cheese melt.

Serve warm directly from the pot or transfer to a casserole serving dish. Garnish if desired.