How to Make It
In a small bowl, whisk together the first 8 ingredients (through Worcestershire sauce). Refrigerate dressing until ready to serve. (The dressing can be made up to 2 days ahead.)
When you're ready to serve the salad, place the spinach in a large salad bowl. Top with the bell pepper, onion, and avocado pieces. Drizzle the dressing over the salad, and toss together gently to combine.