Add chicken broth (or stock), pasta, red onion, fajita seasoning, oregano, salt, and chicken into the Instant Pot without mixing. Make sure the pasta is submerged in the liquid.
Top with the tomato salsa, but do not mix in.
Cover and seal the Instant Pot, select high pressure, the pressure cook (manual) function and cook for 4 minutes.
When done, perform a quick pressure release, open the lid, and then quickly add in the peppers, half and half, and grated cheese. Cover and let stand for 7 minutes, allowing peppers to soften and the cheese to melt.
Open the lid, stir, garnish as desired, and serve.