1 refrigerated pie crust, or homemade pie crust for a single-crust
2 tsp olive oil
1 small onion, diced
10 thin asparagus spears, ends trimmed, cut into 1/4-inch pieces
1/2 medium red bell pepper, seeded, ribs removed, diced
3 oz baby spinach leaves
1/2 tsp dried thyme leaf or 1 tsp fresh thyme leaves or flat-leaf parsley
4 eggs + 1 egg
2 Tbsp Greek yogurt
1/4 cup grated Gruyere cheese
2 Tbsp grated Parmigiano-Reggiano cheese
1/2 tsp kosher salt
1/2 tsp fresh black pepper
Preheat oven to 350°F.
Unroll one pie crust (for a 9-inch pie) and center it in a pie plate. (I used a standard Pyrex® pie plate.) Crimp the edges by hand, or press a design all around with a fork. Set aside.
In a nonstick frying pan, heat the olive oil over low-medium heat. Add the onion, asparagus, and red pepper and cook, stirring occasionally, for 5-6 minutes until the onions are translucent. Add the spinach and thyme, stir to combine, and cook for 2 minutes, until the spinach is wilted. Remove pan from the heat and set aside to cool for at least 15 minutes. (This step is important; if the filling is too hot, it will melt the pastry before the quiche gets to the oven. The filling also can be made in advance and refrigerated.)
In a large measuring cup or bowl, whisk 4 eggs until they are smooth. Add the yogurt, and whisk again. Stir in the two cheeses, salt and pepper.
Scatter the vegetable filling evenly in the pie crust. Pour the egg mixture over the top. Push it around with a rubber spatula so it covers all of the vegetables.
In a small bowl, beat 1 egg with 1 teaspoon of water. Using a brush, paint the crust with the beaten egg.
Bake for 40 minutes. Let sit at room temperature for 10 minutes before serving. Serve warm, at room temperature, or cold.