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.