Gruyère Quiche with Golden Onion and Red Pepper
You can prepare the sautéed vegetables (the first two steps) up to several days ahead.
Place a medium-sized skillet over medium heat and wait about 2 minutes. Add the oil and swirl to coat the pan. Add the onion, sauté for 5 minutes, and then add the salt, herbs, and mustard. Cover the pan, lower the heat, and cook for 15 minutes, stirring occasionally (During this time, preheat the oven to 375°F (190°C), and place the unbaked crust on a baking tray.)
Sir the vinegar and bell pepper into the onions, turn the heat up to medium, and cook, uncovered, for another 5 minutes. Remove from heat.
Sprinkle the cheese into the crust, then spoon the onion-pepper mixture on top of the cheese.
Whisk together the eggs, milk, and black pepper to taste, and slowly pour this over the vegetables and cheese.
Bake on the baking tray in the lower third of the oven for 35 to 40 minutes, or until the custard is set. Cool for at least 10 minutes before slicing, and serve at any temperature.