Preheat oven to 325°.
Combine milk, sugar, and half-and-half in a small, heavy saucepan. Heat mixture over medium heat to 180° or until tiny bubbles form around edge (do not boil), stirring occasionally. Remove from heat.
Combine lime juice, egg yolks, and egg in a medium bowl, stirring well with a whisk. Gradually add the hot milk mixture to egg mixture, stirring constantly with a whisk.
Divide the mixture evenly among 6 (4-ounce) ramekins or custard cups. Place ramekins in a 13 x 9-inch baking pan, and add hot water to pan to a depth of 1 inch. Bake at 325° for 30 minutes or until center barely moves when the ramekin is touched. Remove ramekins from pan, and cool completely on a wire rack. Cover and chill 4 hours to overnight.
Carefully sift 1 1/2 teaspoons sugar over each custard. Holding a kitchen blowtorch about 2 inches from the top of each custard, heat the sugar, moving the torch back and forth, until the sugar is completely melted and caramelized (about 1 minute). Serve the custards immediately.
Note: You can buy a kitchen blowtorch at most cookware stores. But if you don't have one on hand, you can make the sugar topping on the stovetop instead. Place 1/4 cup sugar and 1 tablespoon water in a small, heavy saucepan. Cook over medium heat 5 to 8 minutes or until mixture is golden. (Resist the urge to stir, since doing so may cause the sugar to crystallize.) Immediately pour the sugar mixture evenly over the cold custards, spreading to form a thin layer on top.