Combine the cucumbers, red onion, and avocado slices in a large mixing bowl. Add the chopped coriander and dill if using.
Drizzle the lime juice and olive oil over the salad, then sprinkle the salt, black pepper, and red pepper flakes evenly across the ingredients.
Gently toss everything together using your hands, carefully separating the avocado slices as needed to ensure an even coating of the dressing. Taste and adjust with more lime juice if desired.
Transfer the salad to a serving dish and enjoy immediately.