The sauce. Heat a very large (12-inch) heavy skillet over medium. Scatter the garlic cloves on one side. Turn them regularly until they are blackened in spots and soft, about 15 minutes. While the garlic is roasting, toast the dried chiles, pressing them down with a spatula against the hot surface for a few seconds until they release their aroma and change color slightly on the inside. Toast both sides. Place the chiles in a bowl, cover with hot tap water, weight with a plate to keep them submerged and let rehydrate for 20 minutes.
If using fresh tomatoes, spread them onto a rimmed baking sheet and roast about 4 inches below a preheated broiler until they are blackened and blistered on one side, about 5 minutes, then flip them over and roast the other side. Cool until handleable, then pull of the blackened skins.
Peel the garlic. Drain the chiles and place in a blender jar with the garlic, tomatoes (roasted fresh or from the can), oregano, black pepper and cumin. Blend until smooth, adding a little water if necessary to keep the mixture moving through the blender blades.
Heat a large (4 quart) heavy saucepan over medium-high heat and film the bottom with 2 tablespoons oil. When hot, set a medium-mesh strainer over the top and pour in the tomato-chile mixture. Press it through the strainer into the hot oil; discard the seeds and unblended chile skins left in the strainer. Stir the sauce in the pot nearly constantly until reduced to the consistency tomato paste. Stir in the broth or water, and the masa harina reduce the heat to medium-low and simmer for about 30 minutes.
The filling. Set the skillet over medium heat, film with the remaining 2 tablespoons of the oil and, when hot, add the onion. Cook, stirring regularly, until golden, but still retaining a little texture, about 7 minutes.
While the onions are browning, scoop the potatoes into a microwave-safe bowl, add a splash of water, cover with plastic wrap, poke a few holes in the top and microwave on high for 3 minutes.
Add shrimp and/or scallops to the skillet with the onion and stir until they are just barely cooked through, 4 or 5 minutes, then add the steamed potatoes. Stir and remove from the heat.
Finishing the dish. If the sauce is thicker than a cream soup, stir in a little more broth or water. Taste and season with salt, usually about 1 teaspoon. Keep warm over low heat. Stir ½ cup of the sauce into the seafood-potato mixture.
Brush or spray both sides of the tortillas with a little oil, wrap in a plastic bag (don’t seal) and microwave for 45 seconds. Rewarm the filling if it has cooled off. Lay out 6 tortillas, spoon a portion of the filling down each one, roll up and place on 2 or 3 warm dinner plates. Repeat with the second half of the tortillas. Ladle a portion of the hot sauce over the enchiladas on each plate, then top with some of the tangy greens, a sprinkling of the cheese and some cilantro leaves. Serve without hesitation.