Heat a grill pan over high heat. Place the onions and tomatillos cut-side down and the bell peppers cut-side up on the grill. Grill the vegetables until charred, about 10 minutes. Place in a blender and puree until smooth. Set aside. Heat the oil in large nonstick Dutch oven set over high heat. Add the pork and cook, stirring, until the meat is no longer pink, about 5 minutes. Reduce the heat to medium, add the garlic and cook, stirring occasionally, 1 to 2 minutes. Add the vegetable puree, oregano and bay leaf. Reduce the heat to low and simmer for 10 minutes. Stir in the spinach and cook until wilted, about 1 minute. Season with salt and pepper. Garnish with the diced tomatoes and cilantro leaves to serve.
If you can't find tomatillos, use underripe tomatoes and 1 tablespoon lime juice in their place. Green tomatoes are best, but orange and light green or rock hard ones work as well.
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