1. Preheat oven to 400 degrees Fahrenheit (200 degrees Celsius).
2. In a medium skillet over medium-high heat, heat oil. Add onion and peppers and sauté until onion is translucent, about six to seven minutes. (If you’re using fresh spinach, after about five minutes, add in your chopped spinach until cooked down.) Sprinkle salt on the vegetables as they cook. Set aside to cool.
3. While the vegetables are cooking, make sure the frozen spinach is as dry as possible by squeezing out all the water.
4. In a small bowl, whisk together eggs, almond milk, and nutritional yeast if using. Scatter veggie mixture and squeezed-out chopped spinach into crust. Pour egg mixture over top.
5. Bake uncovered for about 45 minutes. Let quiche rest at room temperature for 10 minutes, then cut into slices.