Prepare the banana caramel: In a heavy-bottom, high-sided saucepan, combine water, sugar and lemon juice (to keep the mixture from hardening). Cook mixture over low to medium-low heat, stirring constantly with a wooden spoon, until sugar dissolves and mixture becomes clear and begins to simmer. If using a thermometer, sugar melts at about 320 degrees F.
After sugar dissolves and mixture is simmering, cook for approximately 8 to 10 minutes over medium-low to medium heat, without stirring at all. To distribute color evenly as sugar caramelizes, hold handle of pan and gently tilt the pan off the heat. It will be done when mixture reaches a golden brown (light amber) color. If using a thermometer, the temperature should register a final temperature of approximately 340 to 350 degrees F. Watch the changing of the color (or the temperature) carefully as it can go past the light brown stage quickly and burn.
Remove pan from the heat, add the sliced bananas carefully, stir mixture gently to cover the bananas in caramel, and add boiling water, if caramel is too thick.
Using a large spoon or ladle, spoon banana caramel into a large (13 x 9-inch) baking dish, distributing bananas in an even layer. Set aside and let cool.
Prepare the custard cream: In a medium to large stainless steel saucepan, whisk cornstarch with a bit of milk until mixture presents no lumps. Place egg yolks in a sieve and strain in order to remove membrane. Add to the cornstarch/milk mixture, whisking/stirring well just until combined. Add the remaining milk and sweetened condensed milk, and stir to combine. Place pan on the stove top (large burner) and cook mixture over medium-low to medium heat, whisking constantly until mixture has thickened and coats the back of a spoon (about 8-12 minutes). Remove pan from the heat, add the vanilla and butter, stirring well until butter has completely melted.
Spoon custard cream in an even layer on top of the banana caramel and let cool. NOTE: Do not use an aluminum pan if using a whisk to stir mixture, otherwise metal will scorch, resulting in metallic contamination.
Preheat oven to 425 degrees F (220 degrees C).
Prepare the meringue: In the bowl of a mixer with a whisker attachment, whisk the egg whites at medium speed until stiff. Add the sugar a third at a time, whisking well between each addition until stiff and shiny. Stir in the lime zest.
Spoon the meringue on top of the custard cream, piling it up in the center and swirling with the back of a spoon in order to form waves and peaks. Bake in the preheated oven just until the top of the meringue becomes golden, about 5-10 minutes.
Serve banana meringue pie warm, or let it cool before chilling in the refrigerator. Although this is a crustless, gluten-free dessert, you can serve with vanilla wafers if desired. Enjoy!