Ceviche Verde With Pepitas

Name: Ceviche Verde With Pepitas
Ingredients: 6 corn tortillas; 1 Tbsp. plus 1½ tsp. avocado oil; ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt; 1 small red onion, thinly sliced (about 1 cup); 1 lb. skinless fillet mild white fish, such as halibut or sea bass; 1½ tsp. Diamond Crystal or ¾ tsp. Morton kosher salt, divided; 2 serrano chiles, stemmed, seeded; ½ cup fresh lime juice (from about 5 limes); ⅓ cup lightly packed chopped cilantro, plus whole leaves for serving (optional); 2 Tbsp. extra-virgin olive oil; 2 Tbsp. mint leaves (from about 2 small sprigs); ½ cup unsalted, roasted pepitas, divided; Freshly ground black pepper; 1¼ cups peeled, seeded, cut into half-moon cucumbers (from about 1 medium English hothouse cucumber or 2 Persian cucumbers); 2 avocados, quartered, sliced lengthwise ¼ thick, divided