Heat your Instant Pot on saute mode. Once hot, add vegetable oil, followed by onion and garlic. Saute for a minute or two, then add ground beef. Cook until the beef is browned, and the onions are translucent.
Remove and drain the beef, then return it back to the Instant Pot. Add in frozen mixed vegetables, Worcestershire sauce, salt, and pepper. Mix well and saute until the vegetables thaw and cook through.
While the vegetables are cooking, add melted butter, flour, and beef broth to a bowl. Mix well.
Pour the mixture over the beef and veggies and stir to mix through. When it begins to thicken, transfer the contents of your pressure cooker to a casserole dish.
Clean the inside of your Instant Pot and add water to it, along with peeled and cut russet potatoes. Salt the water and potatoes and cook on high pressure for 8 minutes. Carefully quick release the pressure when 8 minutes is up.
Drain the water from the pot, then place the potatoes back inside. Using a hand mixer, mash the potatoes until smooth, then add butter and milk. Beat through before adding salt and pepper.
Transfer your mashed potatoes to the casserole dish, spreading over top of the beef and vegetable mixture.
Sprinkle with shredded cheddar cheese, then place under your oven broiler for 5 minutes, or until golden brown. Serve, and enjoy!