If using a frozen rolled pie crust, be sure to set it out on the counter for a few hours so it can thaw and soften.
Preheat the oven to 375 degrees.
Sauté the onions and bell pepper in the olive oil in a medium-sized skillet, over medium heat, for about 5 minutes or until the peppers begin to soften. Add the mushrooms, 1/2 tsp sea salt, dried basil, dried oregano and black pepper to taste. Sauté until the mushrooms begin to release their water, about 5 minutes. Remove the veggies from the heat. Note: this step can be done ahead of time, even the day or two before you are planning to make the quiche. (Leftover cooked veggies work great for quiches!)
When the pie crust is thawed and is soft enough to unroll, place it evenly in a pie dish. Place the veggie mixture on the bottom of the crust.
In a medium bowl, whisk together the eggs, milk and 1/2 tsp sea salt. Pour the mixture over the veggies in the pie dish. Evenly sprinkle the crumbled feta cheese over the top of the egg mixture.
Place the prepared quiche on a large baking sheet, and place the baking sheet and quiche in the oven. Bake for 40 minutes.
Remove your easy veggie quiche from the oven and let it sit for about 5 minutes before serving.