Add the ground beef to the Instant Pot liner. Break it up a bit so it is not a big chunk.
Add onion, garlic, Worcestershire, thyme, bullion, and water (or broth).
Place the long leg trivet into the beef mixture.
Place the potatoes on your steamer and place the steamer basket on top of the trivet.
Secure the lid and set to sealing.
Set to high pressure for 10 minutes.
When the pressure cycle ends, let the pot naturally depressurize for 5 minutes, then quick release the remaining pressure.
Remove the steamer basket and place the potatoes into a large bowl.
Push the saute button and break up the beef.
Add in the frozen veggies and gently combine.
Add in the flour slurry and let saute until the liquid thickens.
Meanwhile, add the milk to the potatoes with salt and pepper to taste. Mash the potatoes.
Assemble your Shepard's pie into desired baking dish putting the beef and veggie mixture on the bottom and the potatoes on top.
Using a butter knife, rub butter on the top of the potatoes and broil until desired potato crispiness.
After approximately 10 minutes, remove the casserole from the oven, allow to cool for 5 minutes, then serve.