The eclairs:
In a thick-bottomed saucepan, bring the water, milk, butter, salt, sugar, and lemon zest to a boil.
When the mixture has reached a boil, pour in the flour all at once and stir vigorously with a wooden spoon for 1 minute, until a ball of dough forms and pulls away from the bottom of the saucepan.
Pour the dough into the mixing bowl, fitted with the whisk attachment, then add the eggs one by one, whisking rapidly.
The puff pastry should have a smooth consistency, but it should not be too soft.

Put the dough in a pastry bag fitted with a smooth tip.

Preheat the oven to 220 C (425 F).

Place 12 strips of dough on a baking sheet lined with parchment paper or a silicone mat.

Bake for 15 minutes, then reduce the oven to 190 C (375 F) and continue baking for 20 minutes.
Remove the eclairs from the oven and let them cool on a wire rack.

Lemon pastry cream:
In a small saucepan, bring the milk and sugar to a boil.
Meanwhile, mix the eggs and cornstarch.
Gradually pour the milk into the egg mixture, whisking.
Transfer the mixture to the saucepan and bring to a boil, whisking constantly, until the mixture has thickened.
Incorporate the lemon curd and mix well.
Cover with plastic wrap and refrigerate.

Cut the eclairs in half lengthwise.
Fill a pastry bag fitted with a fluted tip with cooled cream, then fill the eclairs.

Glaze:
Soften the fondant, then add the pure lemon extract. Mix well and spread over the top of the eclairs.
Keep refrigerated until serving.