Select the high Sauté setting on the Instant Pot and heat the vegetable oil. Add the onion, garlic, serranos, and jalapeño and sauté for about 10 minutes, until the onion is softened and browned. Add the tomatillos, water, and 11⁄2 teaspoons salt.
Secure the lid and set the Pressure Release to Sealing. Press the Cancel button to reset the cooking program, then select the Manual or Pressure Cook setting and set the cooking time for 5 minutes at high pressure. (The pot will take about 10 minutes to come up to pressure before the cooking program begins.)
When the cooking program ends, let the pressure release naturally for 10 minutes, then move the Pressure Release to Venting to release any remaining steam. Open the pot and add the cilantro. Using an immersion blender, puree the mixture until smooth.
Ladle half the sauce into a silicone muffin or loaf pan and slip it into the freezer. When it’s frozen solid, after about 4 hours, pop it out of the pan, transfer to a 1-gallon ziplock plastic freezer bag, and freeze for up to 6 months.
Add the pork to the remaining sauce in the Instant Pot, secure the lid, and set the Pressure Release to Sealing. Press the Cancel button to reset the cooking program, then select the Meat/Stew setting and set the cooking time for 25 minutes at high pressure. (The pot will take about 10 minutes to come up to pressure before the cooking program begins.)
When the cooking program ends, let the pressure release naturally for 15 minutes, then move the Pressure Release to Venting to release any remaining steam. Open the pot, then taste for seasoning and add more salt if needed.
Ladle the chili into bowls and sprinkle with cilantro, pickled onions, and jalapeño slices. Serve piping hot with lime wedges and tortillas or rice.