Preheat oven to 350* F. Spray a 9x13 baking dish with non-stick spray.
Prep veggies: dice the red bell pepper, onion, and spinach into small pieces. Dump them into the prepared baking dish.
Add half of the frozen hashbrowns to the baking dish. Do NOT thaw the hashbrowns ahead of time.
Whisk together the eggs, salt, mustard, and Greek yogurt. Dump the egg mixture into the baking dish.
Add 1.5 cups of cheese (reserving the rest for the top) to the baking dish. Mix it up until evenly mixed. Now add the rest of the hashbrowns and carefully stir until everything is well coated in the egg mixture.
Top with the remaining 1/2 cup cheese. Bake for about 50 minutes, checking occasionally toward the end of the cook time. You'll want the eggs to be fully set and liquid to be evaporated.