Ceviche Tostadas with Ensenada Ceviche

Name: Ceviche Tostadas with Ensenada Ceviche
Ingredients: 1 1/2 pounds skinless, boneless dorado, ((or other fish)); Salt; 1 cup lime juice; 1 medium white onion, (minced); 1 to 4 serrano chiles, (minced); 1 yellow or orange bell pepper, (diced small); 1 teaspoon dried Mexican oregano ((optional)); 4 to 6 Roma tomatoes, (diced small); 1/2 cup chopped cilantro, (or more if you want); sliced radishes, (for garnish); 1/4 cup minced white onion; 1 serrano, (minced); 3 tablespoons lime juice; 2 tablespoons minced cilantro ((optional)); 2 avocados; 12 to 18 corn tortillas