Add 2 tbsp of olive oil to a 9.5in oven-safe skillet and set to medium heat. Once warm, add the mushrooms and cook undisturbed for 3-5 minutes or until lightly browned.
Next chop the scallion and save 2 tbsp of the greenest parts towards the top for later. Then add the rest of the chopped scallions to the pan with the mushrooms.
Cook for 2-3 minutes to soften the scallions and then add the zucchini, tomatoes, 1/2 tsp of kosher salt and 1/4 tsp of black pepper. Cook for one minute.
Next add the remaining 2 tbsp of olive oil the pan along with the hashbrowns and toss to combine with the veggies.
Cook undisturbed the hashbrowns for 5 minutes, sprinkle with salt and pepper if desired, then add the vegan cheese on top. Pour in the vegan egg and transfer to the oven.
Bake for 25 minutes or until golden brown around the edges and set in the center.
Remove from the oven and let rest for 10 minutes before slicing.