Make the sauce. Heat a broiler and position the shelf on its highest setting. On a rimmed baking sheet, spread out the green chiles, unpeeled garlic and onion slices (in a single layer). (You may want to line the sheet with foil for easy clean up.) If using fresh tomatoes, spread them onto the sheet as well; set aside canned tomatoes to add later. Broil until everything is blotchy black and softened, then flip everything else and broil on the other side—tomatoes will take about 6 minutes per side; other vegetables may be ready in less time.
Cool everything until handleable, then pull off the tomatoes’ blackened skin (if you wish) and peel the garlic. Cut the chiles into several pieces, then combine all the roasted vegetables in a blender or food processor and coarsely puree (the mixture should still have a little texture). In a large (4-quart) saucepan, heat the oil or lard over medium-high. When hot enough to make a drop of the puree sizzle sharply, add it all at once. Stir nearly continuously for several minutes until the mixture darkens and thickens nearly to the consistency of tomato paste, about 7 minutes. Add the broth or water and the epazote or cilantro. Simmer for 20 or 30 minutes for the flavors to come together, then taste and season with salt, usually 1/2 teaspoon (depending on the saltiness of your broth).
Finish the dish. Scoop the chopped onion into a strainer and rinse under cold water to rid it of unpleasant flavors, shake off excess moisture and scoop into a medium bowl. Add the crumbled cheese and toss to mix (use a light hand so that it stays light and separate). Either quick-fry the tortillas one by one in oil to soften them, blotting them dry with paper towels, or brush or spray both sides of the tortillas with oil, slide into a plastic bag (don’t seal) and microwave for 1 minute at 100% power. Three at a time, lay out tortillas, scoop a couple of heaping tablespoon of cheese onto each one, splash with a little sauce, then fold in half (like a half-moon) and lay ton a deep warm dinner plate, slightly overlapping. When 4 plates are filled (there should be a some of the filling left), bring the sauce to boil, ladle a portion over each set of filled tortillas, dollop with the crema, then scatter on the remaining filling, the añejo cheese (or a substitute) and the cilantro leaves. You’re ready to eat.