Pat the pork dry with a paper towel and sprinkle with salt. In a large (14-inch) skillet or the removable stovetop safe insert of your slow cooker, heat 2 tablespoons of lard or oil over medium high heat. Working in batches, brown the pork and transfer to a plate when finished, leaving behind as much fat as possible.
In the same pan, add the onion and cook until golden brown, about 8 minutes. Add the garlic and stir until fragrant, then return pork to the pan. In a blender, combine chiles, tomatoes and a little salt and pulse a few times, blending to a coarse puree. Add the tomato mixture to the pan, stir and transfer to slow cooker. Nestle in the epazote or cilantro. Cook on high for 4 hours. Taste and season with salt, usually about 2 teaspoons
Scoop into warm tortillas and garnish with pickled red onions.