Place the flour, salt, butter, and lemon zest in a food processor and pulse until it resembles breadcrumbs. Add the ice water and heavy cream, and process until moist clumps form.
Gather the dough into a ball and knead it for a minute on a lightly floured surface. Divide into 6 equal parts and roll into balls. Transfer the balls to a plate and cover with plastic wrap. Refrigerate for 30 minutes.
On a floured surface, working with one ball at a time, roll the dough ball into a disk larger than your tart tins. Line the tin with the dough: on the base and up the sides. Trim the excess and prick the base with a fork. Refrigerate and repeat with the remaining dough balls.
Preheat the oven to 400 F (200 C). Remove the shells from a refrigerator and line each with aluminum foil. Fill them with pie weights or uncooked rice, beans, etc. Bake until the pastry begins to turn light brown, for about 10-12 minutes. Discard the weights and foil and bake for another 5 minutes.
Meanwhile, prepare the custard by beating the egg together with brown sugar. Stir in the milk, cream, and vanilla.
Pour the custard into the tart shells. Sprinkle with the ground nutmeg. Reduce the oven temperature to 350 F (175 C) and bake the tartlets for another 35 minutes. Remove from the oven and let cool. Then, transfer to a fridge for another 1-2 hours to cool completely.
Arrange the strawberry slices on top and garnish with the fresh mint leaves. Enjoy!