For the Lemon Curd:

1. Put the lemon juice and zest in a pot over medium heat. Add the sugar and stir until it dissolves. Reduce the heat to low, making sure the mixture is below a simmer, and whisk in the butter and eggs.
2. Stir almost constantly, keeping the mixture below a simmer, until it thickens, about 8 minutes. Strain through a fine-mesh strainer into a bowl. Cover with plastic wrap, pushing the plastic wrap directly onto the surface of the lemon curd, and then cool completely. Keep in the fridge until you are ready to use. Leftover curd can be stored in an airtight container in the fridge for up to two weeks.

For the Marshmallow Meringue:

1. Whisk together the egg whites, sugar, and cream of tartar in a heat-proof bowl until just combined (I used my stand mixer bowl).
2. Place the bowl over a pot of gently simmering water. The water should not touch the bowl.
3. Whisk constantly for a few minutes or until the sugar is completely dissolved and the egg whites are warm.
4. Remove the bowl from the heat and place it on your stand mixer (alternatively, use a hand mixer). Beat the mixture with the whisk attachment, slowly at first, and then gradually increase the speed. Beat on high for 5-7 minutes, or until the mixture is thick, glossy, and pillowy. Beat in the vanilla extract until combined.

For the Graham Cracker Crumble:

1. Preheat the oven to 350°F and line a small baking sheet with parchment paper.
2. Combine the graham crackers, oats, and sugar in the bowl of a food processor. Pulse until fine crumbs form.
3. Stream in the butter and vanilla extract and pulse until the mixture looks like wet sand. Pulse in the salt.
4. Spread the mixture out on the prepared baking sheet and bake for 10-15 minutes, stirring once or twice, or until golden brown and toasty.

To Assemble:

1. In small glasses or bowls, place some of the crumble. Top with the lemon curd and then the marshmallow meringue. Repeat layering until all the ingredients are used up, ending with a layer of the meringue on top. If desired, using a kitchen torch, lightly toast the top layer of meringue and then garnish with a little more of the crumble.
2. You can serve these immediately or store them in the fridge until you are ready. They will keep for several days in the fridge.