For the crust, combine flour, brown sugar and salt in a medium bowl and mix together to blend well and break up any lumps of sugar. Add butter in small pieces and rub it in with your fingertips until evenly blended. Tip the crumbly mixture into an 8-inch round tart pan with a removal bottom and press it evenly into the base and up the sides. Bake for 15-18 minutes until golden and fragrant. Transfer pan to a wire rack to cool.
Reduce oven temperature to 325°F
While the crust bakes, make the filling. Combine sugar with lemon zest and salt in a large bowl and rub it together with the back of a spoon until fragrant. Whisk in eggs until smooth. Beat mascarpone with a spoon to smooth it out and make it a bit more fluid, then whisk it into the egg mixture until evenly blended. Whisk in lemon juice.
Once the crust is ready, place it onto a baking sheet to help transfer it back into the oven and immediately pour the filling into the hot crust. Place it back in the oven and bake for 20-25 minutes until the filling is set around the edges with just a slight wobble in the center when you gently shake the pan. Do not over-bake or the filling can become rubbery instead of smooth and creamy.
Transfer pan to a wire rack to cool completely, then cover and refrigerate for 2 hours until thoroughly chilled. Dust with powdered sugar just before serving and slice using a hot dry knife.
For the topping, toss frozen berries with sugar and then let stand for 10-15 minutes to thaw and release some juices. Spoon it over the chilled tart before serving.