If Baking the Casserole the Same Day: Preheat your oven to 350°F.
Grease a 9"x 12" casserole dish with the butter, then add the cubes of concha to the buttered casserole dish.
In a large mixing bowl combine the egg yolks, whole eggs, half-and-half, granulated sugar, almond extract, and the zest of the lime. Whisk these together until smooth.
Pour the custard over the cubed conchas and cover the pan with aluminum foil. At this point, you can either let the mixture sit, covered, overnight in the fridge*; or you can bake it right away.
Just before baking, sprinkle the slivered almonds over the casserole. Cover and bake the casserole for 40 minutes. Uncover and continue baking for an additional 10-15 minutes until golden brown, or until an inserted thermometer reads 165°-170°F.
Remove the dish from the oven and allow it to cool slightly. Before serving drizzle the Lime Glaze (below) over the casserole, then enjoy.
Juice the lime you zested earlier. Add the juice to a bowl along with the powdered sugar. Whisk the two together until no lumps remain.
If you find the glaze is too thick, thin it with a little of milk.
*If soaking the casserole overnight: Uncover the dish to sprinkle on the slivered almonds, then recover before baking.To prevent shattering your dish, be sure to put the casserole dish into the oven while it's heating up and not afterit's already at temperature. If you forget to put it in before you turn the oven on, just let it warm on the counter for at least 30 minutes before putting it into the oven. Leftovers can be stored in a food storage container, in the fridge, for up to three days.