Cook bacon in a large skillet and over medium-high heat, turning occasionally, until evenly browned and crispy, about 10 minutes. Drain on paper towels and crumble when cool enough to handle.

Whisk mayonnaise, green onion, parsley, lemon juice, Worcestershire sauce, salt, and black pepper together in a bowl until smooth.

Stir chicken, tomato, and celery together in a bowl. Add crumbled bacon; pour mayonnaise dressing over top and toss to coat. Cover and refrigerate until chilled, at least 30 minutes.

When ready to serve, remove from the refrigerator and stir. Serve over romaine lettuce leaves and garnish with avocado slices.