The choux buns can be made and frozen before cooking, then defrosted and baked when required.
For the choux buns
Pre-heat your oven to 160°C.
Cover two large ovenproof trays with greaseproof paper.
In a medium saucepan, on a high heat, bring the water, milk, butter, salt and sugar to the boil.
Remove from the stove, and quickly stir in the flour and mix until smooth.
Place the saucepan back on the heat and cook for 2-3 mins until the mixture start to come away from the sides of the pan.
Spoon the mixture into a large heatproof bowl.
Carefully whisk in half of the beaten egg mixture until it becomes smooth again. Whisk in the remaining beaten egg and whisk until it is smooth with a dropping consistency.
Using a spatula, place the mixture into a pipping bag fitted with a 1cm nozzle.
On the baking trays put a small dot of mix into each corner of the tray and lay the baking paper onto it. This will stop the paper being blown around by the fan in your oven.
Pipe small domes approximately 3cm in diameter and 2cm high, leaving a gap of 2 cm between each one. Repeat until you have used all the mixture.
Sprinkle with black onion seeds and bake in the pre-heated oven for 25 minutes until golden and crisp.
Once cooked, place on a cake cooling rack to cool before piercing a small hole on the bottom of your choux buns for the filling.
For the Saint Agur filling
In a large bowl using an electric hand whisk, mix all the ingredients together 2-3 minutes until it thickens slightly.
Place into a piping bag, cut a small hole at the bottom of the bag and fill the choux buns through the hole in the bottom.
Arrange on a plate and serve.
Instead of topping your choux buns with black onion seeds you could sprinkle them with grated parmesan, flack seeds, sesame seeds, poppy seeds or chopped pumpkin seeds.