Step 1

Preheat the oven to 400°F.

Step 2

Melt the butter in a cast iron or ovenproof skillet over medium heat. (If your skillet isn’t ovenproof, transfer everything to a deep dish pie plate to bake it.) Add the onion slices and sprinkle a bit of salt and pepper over them. Cook the onions until they are golden-brown and starting to caramelize, about 10 minutes.

Step 3

Remove the pan from the heat and spread the onions evenly across the bottom. Spread the vegetables evenly over the onions. The dish or pan should look fairly full.

Step 4

In a bowl, use a fork to beat the eggs lightly with the milk, cheese, 1 teaspoon of salt, and 1/2 teaspoon of pepper, just enough to break up the yolks and whites. This is a savory custard mixture. Pour the custard over the vegetables and onions and enjoy watching it fill in all the open spaces.

Step 5

Transfer the quiche to the oven and bake for 45 minutes to 1 hour. Once the surface is lightly brown all the way across, it’s fully cooked. Let the quiche cool for about 20 minutes, then slice into wedges.