In a 6-quart pressure cooker, combine chopped carrots, sweet red peppers, and onion at the bottom.

Carefully place the boneless beef chuck roast on top of the vegetables.

Mix together the canned crushed pineapple, reduced-sodium soy sauce, brown sugar, white vinegar, minced garlic, and pepper in a separate bowl, then pour this mixture over the beef.

Lock the lid on the pressure cooker and set it to high pressure for 40 minutes.

Allow the pressure to release naturally after cooking.

Remove the beef from the cooker, let it cool, and shred it with two forks.

Skim excess fat from the liquid in the cooker, mix cornstarch with water, and stir it into the cooking liquid.

Select the sauté setting and heat the mixture while stirring to thicken.

Return the shredded beef to the cooker, stir to combine, and heat through.

Serve the beef mixture in the lettuce leaves and garnish with sliced green onions.