From https://cooking.nytimes.com/recipes/1016269-quiche-with-red-peppers-and-spinach
Preheat oven to 350 degrees.
Heat oil over medium heat in a large, heavy skillet and add onion.
Cook, stirring, until tender, about 5 minutes.
Add a generous pinch of salt, the garlic, peppers and thyme, cook, stirring often, for 5 minutes
Turn the heat to medium low and continue to cook for 5 to 10 minutes more, until peppers are very tender and sweet.
Turn up heat to medium-high and, a handful at a time, add spinach and stir until each handful wilts.
If there is liquid remaining in the pan turn up the heat and stir while you let it cook off.
Remove from heat and transfer to a bowl. If you are making filling a day ahead, refrigerate uncovered.
Beat together egg yolks and eggs in a medium bowl.
Set tart pan on a baking sheet for easy handling.
Using a pastry brush, lightly brush bottom of the crust with some of the beaten egg and place in oven for 5 minutes.
Add salt (I use ½ teaspoon), pepper, and milk to the remaining eggs and whisk together.
Spread pepper and spinach mixture in an even layer in the crust.
Stir together cheeses and sprinkle in an even layer on top. Very slowly pour in the egg custard over the filling.
If your tart pan has low edges, you may not need all of it to fill the shell, and you want to avoid the custard spilling over.
Place quiche, on baking sheet, in the oven and bake for 30 to 35 minutes, until set and just beginning to color on the top.
Remove from oven and allow to sit for at least 10 minutes before serving. Serve hot, warm or room temperature.
Tip
Advance preparation: The filling will keep for a couple of days in the refrigerator. Keep uncovered so that liquid evaporates (otherwise the liquid will dilute the custard). The quiche will keep for 3 or 4 days in the refrigerator.