Turn the Instant Pot to saute mode. Brown the ground beef, onion, and green pepper; drain off excess liquids if more than 1/4 cup.
Stir in the garlic powder, mustard, ketchup, brown sugar and water; mix thoroughly. Pressure cook on manual medium heat for 5 minutes. Quick release pressure. Reduce heat, and simmer for 15 minutes.
Serve on toasted buns with coleslaw and tator-tots.
Notes
If you have leftovers, store in an airtight container in the refrigerator for 3-4 days or freeze for up to 3 months. You can reheat in the microwave, or add a little water and simmer on the stove over medium-low heat.