Preheat oven to 350°F (175°C) and grease a 9-inch pie dish or 8×8 inch baking dish with butter or oil.

Heat butter or oil in a pan over medium heat and add thinly sliced onion and a pinch of salt. Cook for 6–8 minutes until the onion is soft and translucent, smelling sweet.

To prepare the asparagus, either blanch in salted boiling water for 1–2 minutes until bright green and firm, then transfer to ice water, or roast at 400°F (200°C) for 8–10 minutes until tips brown slightly.

In a bowl, whisk the eggs until smooth and add milk or cream, salt, pepper, and nutmeg. Stir in grated cheese, reserving a handful for the top.

Spread the cooked onions in the prepared dish, scatter the asparagus evenly, and pour the egg mixture over it. Tap the pan to remove air bubbles and sprinkle reserved cheese on top.

Place in the oven and bake for 35–45 minutes, checking at 30 minutes. The quiche is done when the edges are set and the center jiggles slightly.

Let the quiche rest for 10–15 minutes to help the custard set before slicing.