Preheat oven to 350°F.
Rinse and cut seasonal veggies evenly into small pieces. Peel, rinse, and dice onions. If using, slice mushrooms and rinse and chop fresh herbs.
Grate cheddar cheese.
Bring a large pot of water to boil. Add seasonal veggies to boiling water. Briefly boil, about 30 seconds. Using a colander, drain the veggies.
In a large bowl, whip eggs with a fork until well blended. Whisk in dried herbs. Set aside.
Coat medium skillet with non-stick cooking spray. Heat over medium-high heat. Add onions and cook until soft, about 5 minutes. If using mushrooms, add now. Add boiled seasonal veggies. Continue cooking until soft and some of their juices have evaporated, about 5 minutes more.
Coat 9x13-inch baking dish with non-stick cooking spray.
Layer ingredients in the baking dish in the 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 160°F.
If using, garnish with chopped fresh herbs.
Cut into 8 equal-size portions.