Dump Ro-Tel Tomatoes into a colander placed in the sink and let them drain while you chop tomatillos and onion.
Remove husks from tomatillos, wash if needed, cut off end where the stem was, then chop up tomatillos into small pieces.
Chop onion into small pieces.
Put drained tomatoes, chopped tomatillos, and onion, chicken stock, diced green chiles, Mexican Oregano, ground cumin, salt, and fresh-ground pepper into the Instant Pot.
Lock Lid, set Instant Pot to MANUAL, LOW PRESSURE, 20 MINUTES.
While Instant Pot comes to pressure and cooks 20 minutes, chop up enough cooked chicken breast to make 4 cups cooked chicken.
Break apart the cauliflower rice in a package of frozen cauliflower rice and let it start to thaw.
When pressure cooking time is up, use QUICK RELEASE METHOD to release the pressure.
Add cooked chicken and cauliflower rice to the Instant Pot. Set to SAUTE, MEDIUM HEAT, and simmer uncovered, or until soup has cooked down some and flavors are combined.
Add chopped cilantro and lime juice and simmer 2-3 minutes more.
Serve Chicken Tomatillo Soup hot, with more lime and chopped avocado to add at the table if desired.