Preheat oven to 350 degrees F.
In a small bowl, combine walnuts, cranberries, cinnamon, coconut oil, lemon juice and agave until well blended.
Place shells on a cookie sheet and spoon 1 rounded teaspoon of walnut mixture into each shell.
Bake for 9 to 11 minutes, or until tarts are golden brown.
Let cool for 10 minutes.
In a small bowl, mix yogurt and lemon zest.
Spoon one dollop of yogurt mixture on each shell. Serve immediately.