Saute Aromatics: Set the Instant Pot on SAUTE function & set it on HIGH heat. Add 3 tbsp butter, as the butter begins to melt gently release the onion & garlic and saute them until the Onion begins to turn soft. Add salt according to taste to quicken the process. Next, add the sliced mushrooms & saute for 2 minutes.
De-glaze pot: Pour the chicken stock and de-glaze the pot very well scraping the sides & the bottom of the pot with the help of a spatula. This will help loosen any stuck food particles & prevent the BURN error.
Layer Spaghetti: CANCEL the SAUTE function. Now, layer the halved spaghetti in a bird nest or zig zag pattern (refer the steps photos above or video above). DO NOT stack the spaghetti all at once in a single pile. They won't cook. Now, gently push down the spaghetti layer with the help of your spatula so that the noodles are coated with the stock.
Pressure Cook: Close the lid of the Instant Pot, seal the valve and PRESSURE COOK on HIGH for 5 minutes. Once, the pressure cooking cycle is over perform a quick pressure release (QPR) by manually moving the valve from the sealing to the venting position. Open the lid carefully.
De-tangle cooked Spaghetti: Pressure cooked spaghetti will look all clumped up. De-tangle the cooked spaghetti strands with the help of a pair of fork or a tong. CANCEL the Pressure cook function & hit the KEEP WARM mode.
Add Dairy: Now, mix the 1 tbsp flour into the slightly warmed half & half. Add the half & half into the pot and mix well until the dairy well coats the spaghetti. Add the diced softened cream cheese & black pepper powder. Mix well until the cream cheese fully melts. At this point the pasta might get a little dry or look thirsty because the sauce will begin to thicken. Add splashes of the extra stock until the dish begins to cook creamy again.
Chicken & Peas: Add the pre-cooked shredded or diced chicken and the thawed frozen green peas. Stir to mix them well.
Serve: Finally add the shredded or grated parmesan cheese. Mix well until the cheese completely melts. Serve hot with a generous amount of butter toasted bread crumbs. Super Yum! **Note: If you prefer an extra cheesy Tetrazzini top layer than add 2 cups sharp cheddar cheese & 1 cup mozzarella cheese (both not mentioned in the ingredients) on top of the tetrazzini in the Instant Pot. Lock the lid and keep covered for 5 mins. The cheese will melt with the residual heat. Open and serve!