Preheat oven to 375 °F.
Coat a 3 - 4-inch pans with cooking spray and set aside.
Heat oil in a medium skillet over medium-high heat. Add mushrooms and cook until browned. Add onions and cook, stirring often, until tender. Remove from heat and allow to cool.
Whisk eggs, milk and mustard until combined. Add herbs, salt and pepper. Mix in the mushroom mixture and feta.
Ladle into prepared pans.
Bake until golden brown and set, about 30 minutes.
Let stand for 10 minutes before serving.