Turn Instant Pot on and select the 'Saute' function on high.
Add olive oil, butter and saute until onions have slightly softened, about 2-3 minutes.
Now, 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.
Select the 'Cancel' function to reset the cooking function. Add chicken broth, thyme and chicken breasts. Place the Instant Pot lid on the Instant Pot.
Next, select the 'Pressure Cook' function on high and set the timer to 5 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, allow the pressure to nautrally release for at least 10-15 minutes. After, quick release allowing any remaining pressure to release from the pot before removing the lid. Carefully remove the lid.
Once you remove the lid, remove the chicken brests and set aside for now.
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 or other favorite pasta and watch your family come running for more!