In a blender, purée the tomatoes, chipotle chiles, and 1 cup water until smooth.
Set the Instant Pot to Sauté and adjust to More or High. Heat the vegetable oil in the pot, add the onion and garlic, and sauté for 2 to 3 minutes, or until the onion is translucent.
Stir in the cauliflower and tomato-chipotle purée. Season with salt and pepper.
Lock the lid into place and set the steam release valve to sealed. Select Manual or PRESSURE COOK and set the timer for 10 minutes on high.
When cooking is complete, quick release the pressure. Unlock and remove the lid.
Spoon the Cauliflower Tinga onto corn tortillas. Garnish with pickled red onions and chopped cilantro.