1. Whisk together vinegar, mustard, cucumber and salt until well combined and salt is dissolved. Whisk in oil adding a little bit at a time. Refrigerate. A hand blender or food processor can also be used for a creamier dressing.
2. Place salad greens in a salad bowl and top with tomatoes, avocado, green onion, cucumber, and sunflower seeds. Pour dressing over salad. Serve immediately.