Step 1

Turn on a multi-functional pressure cooker (such as Instant Pot®) and select Saute function. Add oil and tip pot to coat the entire bottom. Add ground beef, onion, and red bell pepper. Season with salt and black pepper. Cook and stir until vegetables are soft and beef is no longer pink, 5 to 7 minutes. Drain and discard grease.

Step 2

Add garlic to the pot and cook another 30 seconds. Add vinegar. Stir in water, brown sugar, mustard, Worcestershire sauce, tomato paste, chili powder, and red pepper flakes. Add crushed tomatoes and stir well.

Step 3

Close and lock the lid and close the vent. Select high pressure according to manufacturer's instructions; set timer for 5 minutes. Allow 10 to 15 minutes for pressure to build.

Step 4

Meanwhile, toast hamburger buns and coat with butter.

Step 5

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. Unlock and remove the lid.

Step 6

Serve on toasted buns.