Set a 6-quart Instant Pot® to high saute (see Cook's Note). When hot, add in the oil, scallions, garlic and jalapeno. Cook, stirring, until softened and fragrant, 2 to 3 minutes. Hit cancel.
Add the stock, chiles, lime zest, cumin, oregano, salt and pepper. Mix together. Add in the chicken and stir so everything is coated and covered. Follow the manufacturer's guide for locking the lid and preparing to cook. Set pressure to normal (or "poultry" setting) for 20 minutes.
After the pressure-cook cycle is complete, follow the manufacturer's guide for quick release and wait until the quick-release cycle is complete. Being careful of any remaining steam, unlock and remove the lid.
Set the pot to high saute and cook 12 to 15 minutes, stirring frequently. As you stir the chicken will begin to break apart and shred. You want most of the liquid to absorb, leaving a thick sauce. Squeeze in the juice of the halved limes. Taste the chili and add more salt or pepper if needed.
To serve, ﬁll the lettuce cups with the shredded chicken followed by a few slices of avocado, cilantro leaves and radish rounds.
Settings may vary on your Instant Pot® depending on the model. Please refer to the manufacturer's guide.