Turn on a multi-functional pressure cooker (such as Instant Pot) and select Sauté function. Heat oil in the pot. Cook and stir ground beef, onion, garlic, salt, and pepper in hot oil until beef is browned and crumbly and onion is soft and translucent, about 5 minutes. Stir in tomato paste, Worcestershire sauce, and thyme. Pour in broth. Stir in frozen peas and carrots until well blended. Turn off Sauté function.

Place the collapsible metal trivet in the pot so it rests just above meat-vegetable mixture. Place potatoes on the trivet. Close and lock the lid. Select high pressure according to manufacturer's instructions. Set the timer for 15 minutes if potatoes are on the smaller side or 20 minutes if they are medium or large. Allow 10 to 15 minutes for pressure to build.

Release pressure using the natural-release method according to manufacturer's instructions for 10 minutes. Release remaining pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Open the lid and transfer potatoes to a bowl. Turn off the Instant Pot and carefully remove the hot trivet.

Add half-and-half, butter, and garlic salt to potatoes in the bowl; mash until well blended.

Set an oven rack about 6 inches from the heat source and preheat the oven's broiler.

Pour meat-vegetable mixture from the Instant Pot into an 8-inch square baking dish. Spoon mashed potatoes on top and spread out evenly.

Broil in the preheated oven until the top starts to turn golden and brown, about 3 minutes.