To make the crust, place the walnuts and oats into the bowl of a food processor and process until the nuts are in small pieces. Add the flour, sugar and salt and mix to combine. Add the coconut oil, maple syrup and vanilla and mix until the dough comes together.
Grease an 8-inch tart pan with a removable bottom with some oil. Scoop up some dough and press into the bottom and sides of the pan, using the side of your finger to press into the corners. Use up all of the dough. Place in the freezer for 20 minutes. Preheat the oven to 350°F.
Meanwhile, prepare the curd. Place the cranberries, ¾ cup sugar, brown sugar, zest and juice into a small saucepan over medium-low heat. Once the cranberries start to pop, stir, cover and cook over low heat for 30 minutes. Stir occasionally.
Place the tart pan with the crust on top of a jelly-roll pan and bake for 15 minutes.
When the curd is cooked, uncover and let cool for 10 minutes. Place the cranberry purée into a food processor or blender and process until completely smooth. Return to the saucepan. In a medium bowl, beat the eggs and remaining 2 tablespoons sugar. Heat the cranberry purée over low heat. Scoop up ¼ cup of purée and add to the eggs and whisk well. Add another ¼ cup purée and whisk in. Return to the saucepan and whisk well. Cook over medium-low heat for 4 minutes, whisking every 30 seconds or so, and use the spatula to mix the corners. Let the curd bubble a few times before whisking.
Place the curd into the crust and bake in the oven for 25 minutes, or until the curd is set when you lightly shake the pan. Let cool for 30 minutes and then place in the fridge to chill, for at least 6 hours or overnight.
To make the meringue, place the sugar and water into a small heavy saucepan. Bring to a boil, stirring to dissolve the sugar, and use a candy thermometer to cook the sugar until it reaches 230ºF. While the sugar is cooking, beat the egg whites with an electric mixer until stiff. When the sugar is ready, turn the mixer speed to low and then slowly pour the cooked sugar into the bowl, down the side of the bowl, not directly onto the wire whisk. When all of the sugar has been poured in, turn the mixer up to medium high speed and beat for 1 minute until the meringue is thick and shiny.
Use a silicon spatula to spread the meringue all over the top of the cranberry curd. You can use a blowtorch to lightly brown the top, or place in a 450ºF oven for a few minutes, watching the entire time, until the top browns. Store the tart in the fridge.