For the ceviche: Combine the sea bass, olive oil, cilantro, orange juice, green onions, lime juice, garlic, jalapenos and some salt and black pepper and refrigerate 1 to 2 hours.
For the salsa roja: Heat the oil in a saucepan over medium heat, and then saute the onions, garlic and serrano until the onions are translucent, 2 to 3 minutes. Stir in the tomatoes and oregano and simmer about 10 minutes, and then remove from the heat and let cool slightly. Puree the mixture in a blender until coarsely pureed. Season with salt and black pepper, stir in the parsley, if using, and let cool.
For the tomatillo and avocado salsa: Heat a cast-iron or other heavy-bottomed pan until very hot. Add the olive oil and tomatillos and cook until the skins blister and burst. Stir in the onions, garlic and poblano and cook over medium heat until the onions are translucent and half the tomatillo juice has evaporated, 7 minutes. Transfer to a food processor along with the lime juice and avocado and blend until smooth.
For the tostadas: Heat the oil to 350 degrees F in a small pot. Fry the tortillas one by one until golden and crispy, about 1 minute per side. Transfer to paper towels to dry and sprinkle with the salt, adding more if needed.
For assembly: Spoon a dollop of the salsa roja onto the base of the tostada. Place 2 tablespoons of the ceviche on top, taking care to leave most of the juice behind. Spoon the tomatillo salsa over top and garnish with Chile and Lime Cream, Pickled Red Onions and cilantro leaves. (See Cook's Note)
Whisk together the sour cream, ancho, salt and lime juice and reserve for garnish.
Bring the vinegar, sugar, salt and 1/2 cup water to a boil. Pour the boiling liquid over the red onions and let sit 1 hour. Drain and reserve for garnish.
As an alternative, you can use some of your favorite spices to season the tostadas after they've been fried.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.