In a large mixing bowl or food processor, combine butter and flour until it resembles coarse bread crumbs. In a small bowl, combine the egg yolk, water and icing sugar. Pour egg mixture into flour mixture and pulse until the dough begins to form. Roll pastry into a ball, cover with plastic wrap and refrigerate for 30 minutes. Pre-heat oven to 350F. Remove pastry and place between two sheets of wax paper. Roll until it is about 1/4 inch thick. Drape pastry over pie dish and assemble as needed. Poke the base of the tart with a fork and place in freezer for 10 minutes. Cook pastry for 20-30 minutes or until the dough looks golden. Remove and let cool completely.
Place strawberries and sugar in a small bowl. Mix and let rest for 30 minutes, allowing the juice to develop.
Mix mascarpone, lemon zest and icing sugar together. If the mixture is thick, add a splash of cream or milk. Spread mixture into the prepared tart. Strain strawberries into a small saucepan and add the port. Cook on medium-high heat until the mixture becomes thick and syrupy. It should reduce to 1/2 its size.
Spread the strawberries over the top of the mascarpone cheese and drizzle the port reduction over the strawberries. Cover and place in the fridge until ready to serve. This dessert is best served and eaten the day of, however it can be kept covered in the fridge for up to 2 days.
To make two 4.5 inch tarts, make the following adjustments: