Preheat oven to 400°. Whisk eggs and cashew milk. Chop asparagus, mushrooms and green onion.
Heat olive oil in seasoned cast iron pan. Add mushrooms and sauté for 5 minutes. Add asparagus, green onions, salt and tarragon and sauté until tender, about two minutes.
Whisk eggs and milk gently. Overbeating will add too much air to the frittata, and it will eventually fall as it bakes and a dense texture will result.
Pour the egg and milk mixture over the vegetables. Gently stir with a spatula to distribute eggs and vegetables throughout the pan. Cook on medium-low heat for five minutes.
Transfer the frittata to the oven.
Bake until frittata is puffed, and the centre is still slightly jiggly when you move the pan, about 12-16 minutes. The heat from the cast iron pan will continue to bake the frittata after it is removed from the oven. Place the frittata on a cooling rack to cool.