Turn Instant Pot to sauté. Melt the butter. Salt and pepper the steaks and then sear both sides for 3 minutes. Remove and set on a plate. Add onions into the pot and stir 4 minutes. Add in the garlic.
Pour in the water. Stir the soup mix in. Turn off Instant Pot. Add in the steaks. Pressure cook 30 minutes. Let pressure release naturally for 10-15 minutes. Move valve to venting and remove the lid.
Place steaks on a plate. Turn Instant Pot to sauté. Make a cornstarch slurry. Pour it into the Instant Pot. Add salt and pepper to taste. Turn off Instant Pot.