Follow these simple instructions to create your vegetarian quiche:

1. Preheat the Oven: Preheat your oven to 375°F (190°C).
2. Prepare the Pie Crust: Place the pie crust in a 9-inch pie pan. If using a homemade crust, roll it out to fit the pan and poke holes in the bottom with a fork to prevent bubbling.
3. Sauté the Vegetables: In a skillet over medium heat, sauté the diced onion and bell pepper until soft, about 5 minutes. Add the mushrooms and spinach, cooking until the spinach wilts. Season with salt, pepper, and thyme.
4. Prepare the Egg Mixture: In a bowl, whisk together the eggs and milk. Season with a pinch of salt and pepper.
5. Layer the Filling: Spread the sautéed vegetables evenly in the pie crust. Sprinkle the shredded cheese on top.
6. Add Egg Mixture: Pour the egg and milk mixture over the layered vegetables and cheese, ensuring even distribution.
7. Bake: Place the quiche in the preheated oven and bake for 40-45 minutes, or until the egg is set and the top is lightly golden.
8. Cool: Remove from the oven and allow it to cool for 10-15 minutes before slicing.
9. Optional Garnish: Add halved cherry tomatoes on top for a fresh burst of flavor and color before serving.

These straightforward steps guide you through creating a delicious vegetarian quiche that is sure to impress!