How to Make It
Preheat broiler with oven rack 6 inches from heat. Place baking sheet in oven (do not remove pan while oven preheats).
Add 1/2 cup water, vinegar, and sugar to a saucepan over high; bring to a boil, stirring to dissolve sugar. Place radishes in a heatproof bowl; add vinegar mixture.
Toss onion rings with 1 tablespoon olive oil in a medium bowl. Brush corn with remaining 1/2 tablespoon oil. Carefully remove pan from oven; add onion and corn to pan, and broil until charred, about 15 minutes, turning halfway through. Adjust oven temperature to 400°F.
Combine refried beans, 1 1/2 tablespoons salsa, and 1/8 teaspoon salt in a small saucepan over medium; cook until warm, about 5 minutes.
Cut corn kernels off cob; combine with arugula, cooked green beans, tomatoes, remaining 5 tablespoons salsa, and remaining 1/8 teaspoon salt in a large bowl.
Bake tostada shells on a baking sheet until warmed, about 1 minute. Spread 3 tablespoons bean mixture on each tostada shell. Top each with 3 onion rings, 1/2 cup arugula mixture, 2 tablespoons drained pickled radishes, avocado, plantain strips, and 1 tablespoon cilantro.