In a food processor, puree tomatillos until smooth. In a large Dutch oven or other heavy pot, heat oil over high. Add pork and cook until browned, 9 minutes. With a slotted spoon, transfer pork to a plate. Add onion, poblanos, and jalapenos to pot and cook until onion is translucent, 4 minutes. Season with salt and pepper. Add garlic and cook until fragrant, 30 seconds. Return pork to pot and add tomatillo puree and 2 1/2 cups water. Bring to a boil over high. Partially cover, reduce heat, and simmer until pork is tender, 1 1/2 to 2 hours. Season to taste with salt and pepper.