Put 1 cup of water and the beef into a 6-quart Instant Pot®. Follow the manufacturer's guide for locking the lid and preparing to cook. Set to pressure cook on manual high pressure for 10 minutes (see Cook's Note).
After the pressure cook cycle is complete, follow the manufacturer's guide for quick release and wait until the quick release cycle is complete. Be careful of any remaining steam and unlock and remove the lid.
Turn the pot to the saute function and stir in the onion, garlic, chili powder, cumin, cornstarch, 1 tablespoon salt and 2 teaspoons pepper until combined and the meat is broken up. Let simmer until thickened and the onions are softened, about 8 minutes.
Serve with the taco shells and toppings.
Settings may vary on your Instant Pot® depending on the model. Please refer to manufacturer's guide.
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