In a medium Dutch oven, heat the oil, add the onion, peppers and garlic. Saute until translucent, do not brown. Remove and set mixture aside.
Lightly grill the tomatillos on open flame until lightly charred. Remove from heat, place in bowl and cover with plastic wrap to keep warm for 20 minutes.
Add pork butt to Dutch oven and cook over high heat until browned on all sides
Add the onion-pepper mixture and tomatillos to the pork. Mix thoroughly and then deglaze with white wine and vinegar. Let reduce for 5 minutes, then add chicken stock, oregano, cumin, salt and pepper.
Let simmer for 1 hour.