Place the Ro-Tel can in a colander to drain while you prepare the vegetables.
Remove husks from the tomatillos, rinse if needed, trim the stem ends, and chop into small pieces to yield about 2 cups.
Finely chop the onion.
Add the drained tomatoes, chopped tomatillos, chopped onion, chicken stock, diced green chiles, Mexican oregano, ground cumin, salt, and fresh-ground pepper to the Instant Pot.
Lock the lid and set the Instant Pot to Manual, low pressure (or use the Soup setting) for 20 minutes.
While the pot comes to pressure and cooks, dice enough cooked chicken to measure 4 cups and break apart the frozen cauliflower rice so it can begin to thaw.
When the 20-minute pressure cook finishes, perform a quick release of the pressure according to your cooker’s instructions.
Stir in the diced cooked chicken and the thawed cauliflower rice. Set the Instant Pot to Sauté at medium heat and simmer uncovered for 15–20 minutes, or until the soup has reduced slightly and the flavors are combined. Reduce heat if it boils too vigorously.
Chop the cilantro, dice the avocados, and cut lime wedges while the soup simmers.
Add the chopped cilantro and 1 tablespoon fresh lime juice to the soup and simmer 2–3 minutes more.
Serve the soup hot with diced avocado and additional lime wedges at the table, if desired.
