Grease a 3-quart rectangular baking dish; set aside.
Trim the crusts from the bread slices. Spread a side of 6 of the bread slices with half of the softened butter. Place the bread slices , buttered side up over the bottom of the prepared baking dish, trimming slices as necessary to fit; set aside.
In a large bowl, stir together the tuna, celery, onion, and garlic. Add the mayonnaise and spices; stir to combine all of the ingredients.
Spread the tuna mixture over the buttered bread slices in the baking dish. Spread the remaining butter over a side of each of the remaining bread slices. Place the bread slices, buttered side up, over the tuna mixture, trimming the slices as necessary to fit.
In a large bowl using a wire whisk, beat the eggs. Stir in the milk and undiluted cream of mushroom soup; mix well. Carefully pour the soup mixture evenly over the bread and tuna layers in the baking dish. Cover the baking dish and chill in the refridgerator overnight.
Preheat the oven to 350 degrees. Bake the casserole, uncovered, in the oven about 1 hour or until the top of the casserole is golden brown.