Set an oven rack about 6 inches from the heat source and preheat the oven's broiler. Line a baking sheet with aluminum foil.

Place chile peppers, cut-sides down, onto the prepared baking sheet.

Cook under the preheated broiler until the skin of peppers has blackened and blistered, 5 to 8 minutes. Place blackened peppers into a large bowl and cover tightly with plastic wrap. Allow peppers to steam as they cool, about 20 minutes. Remove and discard skins; chop peppers into small pieces and set aside.

Place pork in a large resealable plastic bag; add flour, 2 tablespoons salt, and 2 tablespoons black pepper. Seal the bag well and toss to coat.

Heat oil in a skillet over medium heat. Cook and stir pork in hot oil until browned, 5 to 7 minutes. Transfer to a slow cooker.

Add chopped peppers, chicken stock, diced tomatoes with green chile peppers, onion, cumin, remaining 1 tablespoon salt, remaining 1 tablespoon black pepper, and garlic to the slow cooker; mix well.

Cook on Low for 6 hours or High for 4 hours, until pork is tender and the flavors blend.