Wash the cucumber and tomatoes thoroughly under cold water.
Chop the cucumber into bite-sized pieces and place them in a large mixing bowl.
Cut the tomatoes into wedges and add them to the bowl with the cucumber.
Slice the red onion thinly and add it to the bowl.
Chop the parsley and add it to the bowl.
Cut the avocado in half, remove the pit, and scoop out the flesh. Chop it into chunks and add to the bowl.
In a small bowl, whisk together the lemon juice, olive oil, sea salt, and black pepper until well combined.
Pour the dressing over the salad ingredients in the large bowl.
Gently toss everything together until the vegetables are evenly coated with the dressing.
Let the salad sit for 5-10 minutes to allow the flavors to meld before serving.