Preheat oven to 350°F and lightly grease a tart pan with a removeable bottom with butter or baking spray making sure to pay attention to the sides.
Combine the oats, walnuts, brown sugar and spices in a food processor. Process until finely chopped then pulse in the cubes of cold butter one by one until the mixture starts to come together.
Turn the ingredients out into the prepared tart pan and press evenly into the bottom of the pan and up the sides.
Place the crust in the refrigerator for 15 minutes.
Remove from the refrigerator, place the tart pan on a baking sheet and bake the crust for 15 minutes.
Remove from the oven, gently press down the middle if it's puffed up a bit from baking and let cool while you make the curd.
Combine the cranberries, lemon zest, 3/4 cup sugar and water in a sauce pot. Bring to a simmer then lower the heat and cook for 15 minutes, stirring occasionally to help break down the cranberries.
Turn off the heat and let cool for 5 minutes then transfer the mixture to a blender and blend until smooth.
Return the mixture to the sauce pot over medium-low heat.
One by one add the remaining ingredients including the leftover 2 tablespoons of sugar to the pot whisking constantly as you go.
Once all ingredients are added to the pot, continue cooking for 10 minutes whisking constantly until the mixture has thickened.
Strain the cranberry curd over a large bowl through a fine mesh strainer using a spatula to push the curd through.
Pour the strained filling into the cooled tart crust and refrigerate for at least 3 hours until set.
Keep refrigerated until serving. Garnish with sugared cranberries or candied lemon slices.
Notes
Keep tart stored in the refrigerator for up to 3 days. Best served within 24 hours of making.