Preheat the oven to 400 degrees F.
In a large bowl, toss the onions, tomatillos, jalapenos, and garlic with 1 tablespoon of the olive oil
and spread on a baking sheet. Roast
until soft and starting to brown, about 20 to 30 minutes, stirring twice during roasting.
Meanwhile, in a large Dutch oven
over medium-high heat, add the oil and heat. Brown the pork, in batches, until well-browned. When browned, add all of the pork back to the pan and cover with chicken stock
. Add the roasted vegetables, cover the pan and place in the oven. Cook until the pork is very tender, about 1 1/2 hours.
While pork is cooking, place the cilantro in a food processor
. Add 2 tablespoons of water and puree. Remove the pork from the oven and stir in the cilantro puree. Season, to taste, with salt and pepper. Serve with warm flour tortillas