In the bowl of a pressure cooker (slow cooker instructions are below), add the onion, stew meat, beef broth, mushrooms, Dijon mustard, Worchestershire, garlic, thyme, and salt and pepper.
Bring instant pot to pressure and cook on high pressure for 10 minutes. Let pressure release.
Make a slurry with the cornstarch and a couple of tablespoons of water; stir in the pot. Stir in the sour cream and cream cheese and choose the sautee button. Cook until mixture thickens a bit. You can also add the egg noodles in at this point with the sour cream mixture if eating right away. If serving later, cook the noodles separately as they will soak up the sauce.
Slow Cooker Instructions: Use Step 1 but cook for 6-8 hours on low. Skip to Step 3 and and the ingredients but cook on low for 30 minutes.