Set the Instant Pot insert in place and select the Sauté setting; let it heat for about 2 minutes.
Add the olive oil, then sauté the diced onion for about 2 minutes until softened.
Add the minced garlic and the bite-size chicken pieces; season with salt, black pepper, Italian seasoning, and dried oregano, stirring and cooking until the chicken is mostly cooked through, about 3–4 minutes.
Press Cancel. Pour in 1/2 cup of the water and scrape the bottom of the pot to deglaze and remove any browned bits to prevent a burn warning.
Add the penne, then pour in the remaining 1 cup of water and the 1 cup of chicken stock; stir to combine and press the pasta down so it is mostly submerged in the liquid.
Close the lid and set the valve to Sealing. Select Manual/Pressure Cook and set the timer for 5 minutes.
When cooking finishes, press Cancel and let sit for 2–3 minutes, then carefully quick-release the remaining pressure by turning the valve to Venting.
Open the lid away from your face and stir the pasta to combine and loosen any sauce.
Stir in the fresh spinach, chopped sun-dried tomatoes, and evaporated milk until the spinach wilts and everything is combined.
Whisk the half-and-half (or whole milk) with the cornstarch until smooth, then stir it into the pasta and mix thoroughly.
Let the pasta sit for about 5 minutes to thicken, then stir in the grated Parmesan until melted and evenly distributed.
Serve warm.
