Rinse and cut seasonal veggies evenly into small pieces. Peel, rinse, and dice onions. If using, slice mushrooms and rinse and chop herbs.
Grate cheddar cheese.
In a large bowl, whip eggs with a fork until blended. Whisk in dried herbs. Set aside.
Coat medium skilled with non-stick cooking spray. Heat over medium-high heat. Add onions and cook until soft. Add finely chopped veggies. Continue cooking until soft and juices have evaporated, about 5 minutes.
Coat 9-by-13 inch baking dish with non-stick cooking spray. Layer ingredients in following order: veggie mixture, egg mixture, cheese, salt and pepper.
Bake until eggs are firm and cheese is melted, about 35 minutes. A thermometer inserted in the middle should read 160F.