Regardless of how many times your spinach was washed, wash it again. Better safe than sorry. Dry it fully by running it through a salad spinner.
Trim the ends of the cucumber and peel it. Then slice it.
Peel and slice the onion into rings. If you are curious about what to do with the other onion half, try this tip for saving avocados. Or if you are just curious as to why onions make you cry, check out the video.
Cut the tomatoes into quarters.
Add all of the vegetables to a large mixing bowl along with the olive oil. Toss the ingredients until all are lightly coated. Salt and pepper to taste.