Start by making your pastry dough. Combine salt and flour in a food processor or mix by hand. Pulse a few times to combine. Add cubed, cold butter and pulse until it resembles sand. If doing this by hand, just break up the cold butter with your fingers and crumble until it reaches a sand like consistency.
Add your egg and pulse until the mixture beings to form a dough, or mix together using a spoon in a bowl, then knead together with your hands to combine. Wrap your dough in plastic wrap and chill in the fridge for about 30 minutes. Towards the end of the chilling time, preheat the oven to 390 F (200 C).
After the dough has chilled, roll out to about 1/4 inch thickness. Prep your mini tart cases by rubbing them with a little bit of oil or butter (these* [this is an affiliate link] are the cases we use). You can also make these in a muffin tin instead! Cut out rounds that are larger than your tart cases and begin to line with the pastry dough. You can also break off pieces of the dough and just press it into the tin and all the way up the sides. It's totally fine if the pastry dough breaks and you have to patch it up! Make sure to press the dough into any ridges in the tart case. Once you've lined the mini pastry cases, trim the edge for a neater look. You can reuse the scraps by pressing them back together and rolling out again!
We got 24 total pastry shells out of this but this may vary depending on the size of your tart cases. You may have to bake in batches!
Once you have lined all the cases, set them on a baking tray. Bake for 18-20 minutes until lightly golden around the edges. Set aside to cool.
While the pastry shells are baking and/or cooling, prepare the filling ingredients. Chop your asparagus into small rounds and finely dice your onion.
Heat up a cast iron skillet with 1 tsp of olive oil and add your chicken. Season with salt and pepper. Cook until your chicken is fully cooked. You can also cook your chicken in the oven on a baking sheet; cook at 400F (205C) for 20-25 minutes, until the internal temperature reaches 165F (74C).
Take out the chicken and let it rest on a cutting board. In the same cast iron skillet, combine your chopped asparagus and onion, garlic powder, and salt and pepper to taste. Cook until the asparagus is tender.
To make the creamy sauce, start by melting 4 tbsp of butter over medium heat in a large pot. Once the butter has fully melted, add 5 tbsp of flour and whisk into the butter. You want to cook this just enough to get rid of any raw flour taste, so just a minute or two.
After about a minute, start adding your milk to the pot slowly, whisking constantly to avoid lumps. Once you have added all the milk, start adding your chicken stock, still whisking continuously. Bring the mixture to a boil for at least a few minutes. You should be left with a creamy and thick sauce. If it is very thick and more paste like, you can add more stock and whisk it in! Make sure to season to taste.
Cut up or shred your cooked chicken. Once the base of the sauce is ready, add your asparagus mixture and chicken and stir to combine.
If you're not going to serve them immediately, you can chill the filling and cover the pastry cases at room temperature until ready to assemble and eat. Just make sure to heat the filling up again before assembling! When you're ready to serve, spoon the mixture into your tart shells. Top with fresh herbs such as parsley and enjoy!
A final note: you may end up with leftover filling! We wanted to make sure there was enough in case the tart cases you are using aren't the exact same size as ours. You can make a wonderful soup with the leftovers by simply heating up the filling and adding more stock!