For the pate a choux, preheat oven to 425°F. Line a baking sheet with parchment paper. 
 
Bring the milk, water, butter, salt and zest to a boil in a medium saucepan over high heat. Reduce heat to medium, add flour and stir with a wooden spoon until the dough forms a ball. Continue to stir and cook for 7 minutes.  
 
Remove to bowl to cool. When just warm enough to the touch, add beaten eggs one-third at a time, mixing swiftly and thoroughly. The dough should be tacky. Scoop onto the prepared baking sheet using a 1 ½-inch cookie scoop (or pipe with a piping bag). 

Sprinkle water onto the parchment (the steam will help the dough to poof) and bake for 25 minutes. Without opening the oven, turn off the heat and leave the puffs inside for 15 minutes to crisp. Remove from oven and let cool completely before filling. 

For the filling, in a medium saucepan, combine the sugar, cornstarch, flour, salt, water, lemon juice and lemon zest. Bring just about to a boil over medium heat while stirring constantly. Reduce heat and stir in butter. Once combined, whisk some of the hot mixture into the eggs to temper them, then pour the mixture back into the pan. Cook over medium heat, stirring often, until thickened, about 3 minutes. Refrigerate in a covered container until chilled.  

For the mousse, make an ice bath. In a small bowl, mix together 1 tablespoon of the lemon juice and the gelatin and set aside.  
 
In a medium saucepan, combine the sugar, cornstarch, lemon zest, remaining 3 tablespoons of lemon juice and salt. Turn the heat to medium, add the boiling water and stir until the sugar dissolves. Mix in the gelatin, add the butter and stir to combine. Cook for a few minutes until the mixture starts to thicken, then transfer it to a medium bowl. 

Place the bowl in the ice bath and whisk to cool and thicken more. Whisk in whipped cream and continue to whisk in ice bath until thick, about 10 minutes. Refrigerate until ready to use, then transfer to a piping bag.  

To assemble, using a sharp knife, slice off the top one-third of each puff, reserving the caps. Fill one-quarter of the way with lemon filling, then pipe lemon mousse on top. Place caps on top.  
 
Garnish with a dusting of confectioners' sugar, berries and mint leaves, if you like.