Sinigang Na Hipon

Name: Sinigang Na Hipon
Ingredients: 1 (33.8-ounce) package fish broth (such as Aneto); 2 medium-size plum tomatoes (about 8 ounces), quartered lengthwise; 1 small (6-ounce) yellow onion, peeled and halved lengthwise; 1 (1-inch) piece fresh ginger, peeled and thinly sliced; 4 medium garlic cloves, smashed; 1 small (11-ounce) red grapefruit; 1 ½ tablespoons fish sauce, plus more to taste; 1 teaspoon fresh lemon juice (from 1 lemon); Fine sea salt, to taste; 12 deveined unpeeled head-on raw large Argentinean prawns (about 1 1/2 pounds) or 1 1/2 pounds deveined unpeeled head-on raw extra-large shrimp; 1 tablespoon seasoning salt (such as Johnny's Fine Foods); 1 tablespoon canola oil, divided; 12 medium shishito peppers (about 3 ounces); Small fresh cilantro leaves and thinly sliced watermelon radish, for serving