In a large stock pot, over high heat, sear the pork in the vegetable oil until golden brown. Remove the pork from the pot, reserve 2 tablespoons oil in the pan, and . Cook the chopped onion and garlic, seasoned with salt and pepper, in the reserved oil until onions are tender. Season with cumin, then stir in pork and chicken stock. Simmer for 1/2 hour.
Stir in poblanos, jalapenos and bell peppers. Puree the tomatillos and cilantro in a blender, and add them to the pot. Cook an additional 30 to 45 minutes.