Prep the produce: Cut the cucumber into half-moons or quarters, depending on thickness. Halve the tomatoes.
Dice the avocado into bite-size cubes. Thinly slice the red onion.
Mix the dressing: In a small bowl, whisk 2 tablespoons olive oil, the juice of 1 lemon or lime, a pinch of salt, black pepper, and a small grated garlic clove if you like.
Toss the base: In a large bowl, combine cucumbers, tomatoes, and onion. Add the dressing and toss gently to coat.
Add avocado last: Fold in the avocado and chopped fresh herbs.
Be gentle so the avocado holds its shape.
Taste and adjust: Add more salt, pepper, or citrus if needed. If using, sprinkle in feta and a pinch of red pepper flakes.
Serve right away: This salad is best fresh, slightly chilled or at room temperature.