1) Preheat the oven to 210C/Gas 7.
2) Roll out shortcrust and mould into an 28cm pie dish, crimping the edges to make ridges. Pierce the shortcrust with a fork several times. Bake in the hot oven until slightly browned, about 6 to 8 mins. Remove from the oven and let cool. Turn the oven down to 160C/Gas 3.
3) In a large skillet over medium-high heat, add the butter, onions, asparagus, cayenne and garlic. Cook for 5 to 6 mins until the asparagus is soft and mixture is fragrant. Remove from the pan to a bowl and let cool.
4) In a large bowl, add the chives, Dijon mustard, eggs, double cream, nutmeg, salt and pepper. Whisk together until well combined.
5) Add the asparagus mixture to the pie shell, then pour in the egg mixture. Top with the cheddar and bake until the eggs are set, about 35 mins. Remove from the oven, cut into wedges and serve.