Step 1
Turn on a multi-functional pressure cooker (such as Instant Pot®) and select Saute function. Add in butter and onion and saute for about 3 minutes. Pour in diced tomatoes with liquid; mix to combine. Add frozen chicken breasts. Cancel Saute mode.
Step 2
Close and lock the lid. Select High pressure according to manufacturer's instructions; set timer for 20 minutes. Allow 10 to 15 minutes for pressure to build.
Step 3
Release pressure carefully using the quick-release method according to manufacturer's instructions about 5 minutes. Unlock and remove the lid. Remove chicken and cut into bite-sized pieces. Add chicken back into the pot. Scatter noodles on top so they don't clump up.
Step 4
Mix mushroom soup chicken soup and milk together in a bowl. Pour over noodles. Close and lock the lid. Select High pressure according to manufacturer's instructions; set timer for 7 minutes. Allow 10 to 15 minutes for pressure to build.
Step 5
Release pressure carefully using the quick-release method according to manufacturer's instructions about 5 minutes. Unlock and remove the lid. Stir in taco seasoning and chicken seasoning salt.
Step 6
Set an oven rack about 6 inches from the heat source and preheat the oven's broiler. Spray an 8x12-inch casserole dish with cooking spray.
Step 7
Transfer 1/2 of the spaghetti mixture to the casserole dish and sprinkle with 1/2 cup cheese. Add remaining spaghetti mixture and sprinkle with remaining 1 1/2 cups cheese.
Step 8
Broil in the preheated oven until the cheese is bubbling and golden 3 to 5 minutes.