Turn Instant Pot on and select the 'Saute' function on high.
Add olive oil, butter and meatballs and saute until meatballs are slightly browned on the sides. You do not need to fully cook the meatballs as they will continue to cook during the pressure cooking stage. NOTE: You will want to move the meatballs in the pan regularly so they do not stick to the bottom. Doing so will help you avoid the 'Burn' notice later.
Once meatballs are slightly browned, use a ladle or spoon to scoop them out of the Instant Pot into a bowl. Set aside for now.
With the oil and butter left in the Instant Pot, add onions and saute until slightly softened, about 2-3 minutes.
Add mushrooms, stir and continue to saute with the onions for an additional 3-4 minutes.
Add garlic, stir and continue to cook for an additional 1-2 minutes.
Add Dijon mustard, stir and cook for 1 more minute.
Add beef broth, thyme and meatballs you had set aside. Place the Instant Pot lid on the Instant Pot. Select the 'Cancel' function to reset the cooking function.
Next, select the 'Pressure Cook' function on high and set the timer to 7 minutes. Press 'Start.' NOTE: It will take the Instant Pot about 10 minutes to come to pressure before the cooking time begins.
Once the timer beeps, quick release allowing all pressure to release from the pot before removing the lid. Carefully remove the lid.
Once you remove the lid, select the 'Cancel' function and then select the 'Saute' function on high. Allow the mixture to come to a bit of a boil.
Mix in the cornstarch slurry and stir immediately. Next, add the onion mixture and continue to stir.
After 2-3 minutes, select the 'Cancel' function and then the 'Warm' function to slightly cool the mixture.
After 3-4 minutes, add in sour cream and Boursin and stir until all ingredients are mixed well.
Serve over egg noodles and watch your family come running for more!