Put the oil in a large pot or deep skillet over medium-high heat. When it’s hot, cook the onion, bell pepper, and garlic until the onion is soft and translucent, 5 to 10 minutes. Sprinkle with salt and pepper.
Add the tomatillos and cook until the skins start to break open, 10 to 15 minutes. Pour in the stock and stir. Bring to a boil, then reduce the heat so the mixture bubbles very gently. Cover; cook until the tomatillos are mostly dissolved, about 30 minutes. (You can prepare the dish up to this point, cover, and refrigerate for a day or 2 before proceeding.)
Return the heat to medium-high, and when the mixture starts to bubble, stir in the tomatoes. Cook, stirring occasionally, until the tomatoes wilt, but their skins are still intact. Stir in the cilantro, taste, and adjust the seasoning. Serve hot or at room temperature, drizzled with additional oil.