Preheat broiler. Place toasted English muffins halves on a baking sheet.
Mix together the mayo and garlic. Spread mayo onto each muffin half. Top with turkey, cheese, and onion. Season with salt (sparingly, as lunchmeat is already salty) and pepper.
Broil until cheese is melted and lightly browned, about 2 minutes (give or take, watch carefully).
Top with avocado slices and serve.