Make the meat mixture: In a large bowl, mix 18 oz lean ground beef, 1/2 tbsp olive oil, 1 medium egg, 1/2 tsp salt, 1/2 tsp black pepper, 1 tsp garlic powder, 1 tsp onion powder, 1 oz onion, 1/2 tbsp Worcestershire sauce, and 1.5 oz breadcrumbs until just combined. Let it sit for about 10 minutes.
Form the loaf: Shape the meat mixture into a loaf and place on a piece of aluminum foil.
Make the sauce: In a small bowl, whisk together 2 tbsp ketchup, 1/2 tbsp tomato paste, 1/2 tsp brown sugar, and 1/2 tsp mustard. Set aside.
Set the Instant Pot to ‘stew meat’ mode, add 20 oz potatoes and 1/2 cup water to the pot. Place the wire rack over the potatoes and place the meatloaf on top of the rack. Secure the lid and cook for 15 minutes.
Once done, quickly release the pressure and carefully lift the meatloaf out of the pot. Brush the glaze all over the loaf.
Remove the potatoes from the pot. Switch to ‘sauté mode’, heat 1 tbsp unsalted butter and 2 tbsp garlic until fragrant. Add potatoes and stir fry, about 1 - 2 minutes. Remove from heat.
Sprinkle 2 tbsp chopped scallions on the meatloaf.
Slice the loaf. Serve warm with potatoes.