Combine. Place cucumber, cherry tomatoes, and red onion on a large plate or salad platter. Gently toss together to combine then spread into a single layer. Arrange the avocado slices, tucking them into the salad here and there.
Season. Drizzle the salad with the olive oil, squeeze 1/2 of the lemon over everything, season generously with salt and black pepper. Sprinkle chopped herbs over the top.
Taste. Taste and add more lemon, salt, and pepper if needed. Serve immediately.