In a generous sized skillet, sauté onion and garlic with some olive oil over low-medium heat just until translucent - set aside.
Add mushroom to skillet (with a touch more oil if necessary) and sauté over warmer heat until moisture is evaporated and mushrooms are seared.
Meanwhile, in a separate large mixing bowl, whisk together eggs, salt, milk and half and half. Add cheese if using and whisk again.
Return onion/garlic mixture to skillet with mushrooms and add whisked egg mixture.
Using a spatula, gently fold eggs in the skillet over low'ish-medium heat (being careful not to burn the bottom) - the idea here is not to cook the eggs but gently move them around so that they just start to stick to the surface of the pan (as you move the spatula across base you should start to see a parting) add spinach and arugula to skillet and continue to fold just until greens are integrated. Remove pan from heat while egg mixture is still mostly soft/runny.
Carefully transfer the mixture into a greased ovenproof dish (I used 8 x 11 -- 9 x 9 works well too).
Place in oven and bake uncovered for 25-30 minutes, until egg has puffed and is beginning to turn golden around the edges.
Carefully remove the egg bake from oven, allowing it to cool before serving.
Garnish with cilantro sprigs and serve with salad, as desired.
Recipe by Inspired Edibles at https://www.inspirededibles.ca/2018/05/spinach-mushroom-egg-bake.html