In a large bowl, combine oil, tahini, lemon juice and garlic. Season with salt and pepper, then whisk until well combined.
Add chickpeas and mash with the back of a fork or spoon until chunky.
Add celery, scallions and cheese, and mix well. Season with salt and pepper.
Spread one side of each bread slice with mayonnaise.
On 4 slices of bread, divide lettuce, alfalfa and avocado, then top with some of the chickpea salad and tomatoes.
Close sandwiches and serve. (Sandwiches can sit at room temperature for 1 to 2 hours before serving.)
