Set the Instant Pot to sauté mode and heat the vegetable oil. Add the lamb pieces and brown them on all sides for about 5 minutes. Remove the lamb and set it aside.
Add the diced onion, garlic, and ginger to the pot. Cook for 2 minutes until the onion softens.
Stir in the red curry paste and cook for 1 minute to release its aroma.
Pour in the coconut milk and beef broth. Return the lamb to the pot and stir to combine. Seal the lid and cook on high pressure for 25 minutes.
Quick-release the pressure. Open the lid and add the carrots, red bell pepper, and zucchini. Set to sauté mode again and simmer for 5 minutes until the vegetables are tender but still hold their shape.
Stir in the fish sauce, lime juice, and Thai basil leaves. Season with salt and black pepper to taste. Turn off the pot and let the stew rest for 2 minutes before serving.