If using fresh tomatillos, cover with water in a saucepan, bring to a boil, reduce the heat and simmer 10 minutes. Drain and place in a blender or a food processor fitted with the steel blade. If using canned tomatillos, drain and place in the food processor or blender. Add the chile(s), onion, garlic, about 1/2 teaspoon salt and half the cilantro. Blend until smooth.
Heat the oil in a heavy saucepan or in a nonstick skillet over medium-high heat. Drop in a bit of purée; if it sizzles, add all of the purée to the pan. (Wait a few minutes if the pan isn’t hot enough.) Cook, stirring often, until the purée thickens and leaves a canal when you run a spatula down the middle. Add the stock, stir together, bring to a simmer and simmer, stirring often, for 15 to 20 minutes until the mixture has thickened and reduced slightly. Remove from the heat, taste and adjust the salt. Transfer about 1 cup of the sauce to a blender, and add the remaining cilantro. Blend until smooth, then stir back into the pan (the sauce will now be brighter). Keep warm while you cook the fish and spinach.
Heat 1 inch of water to a boil in the bottom of a steamer. Place the spinach in the top part of the steamer, in batches if necessary, and steam until the leaves have collapsed, about two minutes. Remove from the steamer, rinse with cold water, squeeze out extra water and chop. Set aside.
Add the fish fillets to the steamer, and steam 5 to 10 minutes (depending on the thickness) until the fish pulls apart easily. Remove from the heat, and allow to cool until you can handle it. Flake the fish, using your fingers or a fork, removing all bones as you do so. Stir the fish and the spinach into the green salsa. Taste and adjust salt.
Wrap the tortillas in a heavy kitchen towel, and place in the steamer basket. Bring back to a boil, cover the pot and steam 1 minute. Turn off the heat, and allow to sit for 15 minutes without uncovering. Pass the tortillas, top with the fish and spinach mixture, sprinkle with a little queso fresco if desired, fold the tortillas over the filling and enjoy.