Preheat oven to 400°F and spray a 9x13 baking dish with cooking spray.
Add frozen shredded hash browns (no need to thaw) to baking dish and drizzle with 1 teaspoon oil and a dash of salt and pepper. Bake for about 30 minutes until potatoes start to brown and crisp up. Once you pull potatoes from the oven, press down with a spatula to help form the bottom crust more.
While potatoes are baking add 2 teaspoons oil to a large skillet over medium-high heat. Once hot, add onion, garlic, bell pepper and mushrooms. Cook until onions are translucent and all the liquid from the mushrooms has evaporated, about 7-10 minutes.
Add spinach, salt and pepper to the mixture and stir until spinach has wilted.
Once veggies are cooked, take off heat and set aside to cool.
Whisk eggs and milk in a large bowl.
Once veggies have cooled and are no longer hot, add to egg mixture along with 1 cup shredded cheese and stir to combine.
Pour egg mixture into the baking dish on top of the hash browns and top with additional 1/2 cup of cheddar cheese.
Bake for 20-25 minutes until set and eggs are no longer jiggly.
Let stand for 5 minutes and serve.