Preheat oven to 350 degrees Fahrenheit.
In a small bowl, add eggs, cheese, lemon zest, and seasonings. Mix well to combine, and set aside.
In a cast iron pan heat oil over medium-high heat and add in asparagus. Cook for 3-4 minutes, stirring often.
Next, add in onions and mushrooms. Cook an additional 3-4 minutes, stirring often until asparagus has softened to your desired likeness. Remove pan from heat.
Add egg mixture to the pan and put in the oven for 30 minutes, or until knife comes out clean. Serve.