For the sweet tart dough: 

There's a post for this Sweet Shortcrust Pastry you can check out for more details and images.

1. In a large bowl, cream the butter with a spatula or wooden spoon until it is smooth.
2. Add powdered sugar and mix until all is incorporated.
3. Add egg and mix with a fork. It will not be smooth or completely incorporated. That is fine.
4. Add half of the flour and the salt.
5. Mix it but don’t wait until the flour is completely incorporated before adding the second half.
6. Now, mix it until lumpy and a few dry spots remain.
7. Turn it onto a clean surface or counter and mix it with your hands, making it come together but touching it as little as possible. Don't overmix it. It should not be kneaded, only gathered into a disc or ball. 
8. When you have a smooth dough ball, wrap it in plastic and refrigerate for at least 1 hour and up to 2 days.
9. When ready to roll, take the dough from the fridge and let barely soften at room temperature. The dough must still be cold. Simply wait until you can start to roll it, but not until it is very soft. This is important to ensure a tender dough.
10. On a lightly floured counter, start rolling the dough disc, making sure you do quarter turns (I do them to the left) often to ensure the dough is not sticking and adding more flour to the counter if it is sticking.
11. Once you have the dough rolled a few inches larger than the pan, fold it in half and place the fold in the middle of the pan.
12. Open the dough and line the pan, being careful not to stretch the dough. 
13. Remove excess dough by rolling over the rim of the pan with the rolling pin.
14. Prick the bottom with a fork, freeze for 20 minutes, or put it in the refrigerator for 1 hour. 
15. Preheat the oven to 350ºF /180ºC.
16. Line the tart pan with a piece of aluminum foil and add beans or pie weights, covering the bottom evenly.
17. Bake the tart shell for 15 minutes.
18. Remove the foil carefully and continue baking for 10 to 15 minutes more until the dough is fully baked.
19. Let cool completely on a wire rack. Remove from the pan and transfer to the serving plate.

For the filling:

1. Put the strawberries with the sugar in a bowl (I like glass), mix a little, cover with a kitchen towel, and leave to marinate at room temperature for at least 1 hour, until the fruit juices are fully released.
2. Put the tablespoons of cold water in a small cup or bowl.
3. Sprinkle the teaspoon of unflavored gelatin and let it hydrate for a few minutes. Do not stir.
4. In a small saucepan, put all the juice that the strawberries released, reserving the fruit for later. Heat over low heat until it’s about to break to a boil. Remove from the fire.
5. Add the cold hydrated gelatin to the hot strawberry juice and mix until dissolved.
6. Transfer to a bowl and let thicken a bit, either at room temperature or in the refrigerator (stirring it often) until it begins to thicken, but don't let it set.
7. In a large bowl, beat the heavy cream with the sour cream and powdered sugar until it is of medium thickness.
8. Spread in the bottom of the cooled, baked crust.
9. Carefully distribute the macerated pieces of strawberry left from before, spreading over the entire surface.
10. Slowly and carefully add the thickened gelatin syrup on top of the strawberries, trying to cover as much as possible.
11. Let the cake set in the refrigerator for 15 minutes, garnish with mint leaves and serve.