Heat broiler. Toss the beans, 2 tablespoons of the oil, and ¼ teaspoon each salt and pepper on a rimmed baking sheet. Nestle the fish in the beans and season with the coriander and ¼ teaspoon each salt and pepper.
Broil until the beans are crisp-tender and the fish is just opaque throughout, 6 to 8 minutes.
Meanwhile, combine the avocados, vinegar, dill, the remaining tablespoon of oil, and ¼ teaspoon salt in a medium bowl.
Serve the fish and beans topped with the avocado mixture.