In a large bowl, add 24 oz skinless boneless chicken breasts into a mix of 2 tsp Italian seasonings, 1/2 tsp garlic powder, 1/2 tsp onion powder, 1/4 tsp salt, 1/4 tsp ground black pepper, 1/4 tsp paprika, and 1 tsp Dijon mustard. Toss to distribute the spices as evenly as possible.
Set the Instant Pot to the “Sauté” setting and wait until it's hot. Sear chicken breasts in 1 tbsp olive oil for 7 minutes on both sides. Remove chicken from the pot and set aside.
Add 1 tbsp unsalted butter, 6 oz cremini mushrooms, and 2 tsp minced garlic to the Instant Pot. Sauté until fragrant.
Continue to pour the 1 cup unsalted chicken broth into the pot. Lay a steaming rack over the top and place the seared chicken breasts inside. Close the lid and set the valve to seal. Turn on the “Steam” setting and cook for 7 minutes.
When the time is up, allow the Instant Pot to release the steam naturally for 10 minutes. Open the lid and remove the chicken.
To make the sauce: Set the Instant Pot back to the “Sauté” setting. Add 1/4 tsp salt, 1/4 cup heavy cream, 0.25 oz shredded parmesan cheese, 4 oz green peas, 1/4 cup whole milk, sautéed mushrooms, and 1 tbsp lemon juice.
Stir constantly while the sauce is cooking. Watch as it thickens (about 10 minutes). Add 11 oz cooked spaghetti and let it cook for 5 more minutes.
Turn off the Instant Pot and remove the inner pot.
Transfer the chicken breasts to a cutting board or serving platter. Garnish with 2 tbsp parsley and 0.25 oz shredded parmesan cheese. Enjoy the chicken with spaghetti and mushroom sauce on the side.