Press the Sauté button on the Instant Pot and heat oil. Add onion. Stir-fry 3–5 minutes until onions are translucent.
Add potatoes, curry paste, red chile flakes, garlic salt, pepper, and broth to pot and stir to combine. Press the Cancel button. Lock lid.
Press the Manual or Pressure Cook button and adjust time to 15 minutes. When timer beeps, let pressure release naturally until float valve drops. Unlock lid.
Ladle a spoonful of liquid from pot into a small bowl. Whisk in cornstarch until smooth. Add this slurry back to pot and let sit 5 minutes until sauce thickens.