For the custard: In a small saucepan, bring the milk and lemon zest to a boil over medium heat. Remove the pan from the heat and allow the mixture to cool for 5 minutes.
In a medium bowl, whisk together the sugar, flour and egg yolks (the mixture will be thick). Pour the milk mixture into the bowl, whisking until smooth. Return the mixture to the saucepan and whisk slowly and constantly over medium-low heat until smooth and thick, 6 to 7 minutes. Remove the pan from the heat and stir in the butter. Whisk in the lemon juice and vanilla until smooth. Transfer the custard to a clean bowl and lay a piece of plastic wrap directly onto the surface. Refrigerate for 2 hours.
For the meringue: In a small saucepan, combine the sugar and 1/4 cup water over medium-high heat. Stir until the sugar has dissolved. Boil, without stirring, until a candy thermometer registers 240 degrees F, 3 to 4 minutes. Let the syrup cool for 1 minute. While the sugar mixture is cooking, in a stand mixer fitted with a whisk attachment, beat the egg whites on medium speed until frothy. Increase the speed to high and beat until the egg whites hold soft peaks. Reduce the speed to low and carefully drizzle the hot syrup down the side of the mixing bowl. Beat at high speed until the egg whites hold stiff, glossy peaks, about 2 minutes. Gradually beat in the lemon juice.
Fold the custard into the meringue, then spoon into serving bowls or cups. Refrigerate for at least 1 hour before serving.
For the topping: Position a rack in the center of the oven and preheat the oven to 350 degrees F. Line a small baking sheet with parchment paper. In a food processor, blend together the flour, brown sugar and butter until the mixture begins to form into clumps, about 20 seconds. Add the cookies and pulse until the cookies are crushed into 1/4-to-1/2-inch pieces. Pulse in the almonds until combined. Arrange the mixture in a single layer on the prepared baking sheet. Bake until the almonds begin to brown, 13 to 15 minutes. Cool for 15 minutes.
Sprinkle the topping over the meringue custard and serve.
Food Network Kitchens suggest caution in consuming raw and lightly-cooked eggs due to the risk of Salmonella or other food-borne illness. To reduce this risk, we recommend you use only fresh, properly-refrigerated, clean, grade A or AA eggs with intact shells, and avoid contact between the yolks or whites and the shell. For recipes that call for eggs that are raw or undercooked when the dish is served use shell eggs that have been treated to destroy Salmonella, by pasteurization or another approved method.