1. For the crust: cream the butter and sugar until light and fluffy. Add the egg yolk and vanilla and mix well. Add the flour and stir just until the dough forms. Gather the dough into a ball and break off a ¼ of it to reserve for the lattice topping. Wrap it in plastic and set aside. Press the remaining dough into a 6-inch tart pan. Place the tart pan and the wrapped dough in the fridge to chill for at least 30 minutes.

2. For the filling: Combine the berries, rhubarb, sugar and cornstarch in a small bowl and stir well.

3. Preheat the oven to 375ºF. Pour the prepared fruit into the chilled crush and and roll out 6 thin strips to make the lattice. Lay three strips across in one direction, then basket weave the remaining three ropes in the other direction. Bake for 20 – 25 minutes, or until golden brown.