A rich braised stew of pork and potatoes in a chilli paste that becomes deeper and more mellow the next day.
To make the Chilli Sauce:
In a dry heavy skillet over medium heat, toast the chillies a few pieces at at time, pressing them flat against the hot surface with a metal spatula, until they are aromatic, about 10 seconds per side. If the heat is right, you'll hear a slight crackle when you press them down, but you shouldn't see more than the slightest wisp of smoke. The inside surface of the chilli should look noticeably lighter.
Transfer toa bowl and add hot water to cover. Place a small pate on top to keep the chillies submerged and let stand for 20 minutes.
Using tongs, transfer the chillies to a food processor or blender. Add enough water from eh bowl to make one cup. Add the tomatoes and garlic, and process to form a smooth puree. Set aside.
To make the Filling:
In a 4 quart sauce pan set over medium high heat, warm the oil. When the oil is hot, add the pork in a single layer (in batches if necessary) and cook, turning, until richly browned all over, about 10 minutes.
As the pork is browned, transfer it to a plate. When all of the pork is browned, return it to the saucepan.
Push the chilli tomato puree through a mesh sieve directly into the pan and cook, stirring frequently, until the saucy mixture is as thick as tomato paste, 6-8 minutes.
Stir in the reserved water from making the chilli paste, as well as enough water to make 2.5 cups in total. Add 1 tsp salt, pepper to taste, and cover partially. Bring to a rolling simmer. Reduce to the heat to medium low and simmer, stirring frequently, for 20 minutes.
Add the potatoes, cover partially and cook, stirring frequently until the potatoes and meat are tender but not falling apart, and the sauce has thickened to the consistency of canned tomato sauce, 20-30 minutes more.
If the sauce is too thin, continue cooking, uncovered, until the liquid has reduced a bit more. Taste and adjust the seasonings with more salt and pepper if needed.
Scoop the hot pork and potato mixture into a serving bowl and garnish with cilantro. Serve with warm tortillas. Ole!
Adapted from Mexico One Plate at a Time by Rick Bayless