Pat the chuck roast dry with paper towels. Season on all sides with salt and pepper.
Put the Instant Pot on “Saute”. Once it says “Hot”, add the oil.
When the oil is shimmering, add the chuck roast. Sear the meat until brown, about 3-5 minutes per side.
Add the onion, carrots, and beef broth to the pot. Mix around until evenly distributed.
Set the pressure knob to “Sealing” and close the lid until it beeps. Tip: Closing the lid can be tricky. Have the pressure knob at 11 o’clock and turn right.
Select “Manual” and increase to 50 minutes. It will say “On” in a few seconds. Note: the Instant Pot will need to warm up for 5-10 minutes. Steam will be released and a few minutes later, the time will start counting down.
After the time has expired, let the pot roast rest in the pot for 10 minutes. It will count up and read “L 0:10”.
Using a towel or mit, release the pressure knob from “Sealing” to “Venting”. After the steam has been released, carefully take out the roast and veggies and transfer to a plate or cutting board, only leaving the liquid in the pot. Cover the roast and veggies with foil.
Set the Instant Pot to “Saute” and once the liquid comes to a boil, add the flour. Whisk until mixed in and sauce has thickened, about 1-2 minutes.
Hit “Cancel” and remove the pot. Pour the sauce into another small bowl and cover to keep warm. Then clean out the Instant Pot.
Place the cauliflower florets in the pot. Pour in the beef broth.
Seal the lid and set the pressure knob to “Sealing”.
Select “Manual” and set time to 5 minutes.
Once time is up, using a towel or mit, release the pressure knob from “Sealing” to “Venting”. After steam has been released, carefully remove pot and dump out the liquid.
Using a potato masher or fork, mash the cauliflower until smooth.
Add the salt, pepper, garlic, and sour cream. Stir until everything is combined.