Special equipment: an immersion blender
In a small pot, combine the annatto seeds and 4 tablespoons of the vegetable oil. Cook over medium heat until the oil is bright red. Strain the oil, discard the annatto seeds and set the oil aside
To make the sofrito, combine the peppers, onions, garlic and cilantro in a food processor and pulse until everything is chopped finely.
Pour the annatto oil into a medium pot and heat over medium heat. Add the sofrito to the pot and saute over medium heat until the vegetables are tender. Pour in the chicken stock and coconut milk, add a pinch of salt and bring up to a boil. Reduce the heat to a simmer and cook for 15 minutes. Add the corn kernels and cook for another 5 minutes. Keeping the mixture over the warm stove, blend with an immersion blender until smooth. Strain the mixture through a fine sieve, pressing the pulp with a spatula to get all the liquid out. Season the liquid with a little salt and pepper and set aside.
Heat a medium saute pan over medium-high heat until hot. Working in batches and adding more vegetable oil if necessary, add one tablespoon of the vegetable oil to the pan and then place the tomatillo slices in 1 flat layer on the bottom of the pan. Sprinkle them with a little salt and sear quickly to create a golden surface. Flip the slices and sear the other side. Remove from the heat and set aside.
Season the fish on both sides with salt and pepper. Heat 2 nonstick saute pans over high heat. Divide the remaining vegetable oil between the pans. Once the oil is almost smoking, add 2 pieces of fish to each pan, skin-side down. Immediately press down on each piece of fish with a spatula so the skin is in contact with the pan. Reduce the heat to medium high and continue to cook until the skin is crispy. Flip the fish and turn off the heat. The fish will finish cooking with the residual heat in the pan. Check the doneness of the fish by inserting a metal skewer in the thickest part of the fish, letting it stay in for 5 seconds, and then removing it. If the skewer is warm, the fish is done.
To serve, arrange the tomatillos in rings on each plate. Place the fish on top of the tomatillos. Reheat the sauce for a moment and whir for a few seconds with the immersion blender to give it a foamy appearance. Spoon the sauce around the circle of tomatillos, covering the base of each plate. Finish with a squeeze of lime over the sauce and serve immediately.
Annatto seeds are from the achiote tree and are often used not only for their flavor, but also for the reddish hue they impart to food. You can find them in Mexican supermarkets and specialty food stores. Aji dulce is a variety of sweet pepper that resembles the much spicier habanero. It is used widely in Latin American and Caribbean cooking. You can find it in specialty food stores.