Dice onion, tomatillos, and chicken.
In a large dutch oven, heat olive oil until shimmery, then add chicken.
Brown 3-4 minutes, occasionally stirring, to prevent burning or sticking.
When chicken has browned lightly on all sides, stir in onions and tomatillos and brown another 8 minutes, stirring occasionally.
Add salt, pepper, cayenne, cumin, and oregano. Mix well.
Slowly stir in stock, ro-tel, and drained pinto beans. Stir well and bring to a rapid boil.
Reduce heat to low, and let reduce slightly, about 8 minutes.
Taste soup to see if more salt, pepper, or cayenne are needed.
Serve in bowls with fresh sliced jalapeños, chopped cilantro, and optional - sour cream or shredded cheese.