Melt butter in a skillet. Add in onion and cook until translucent. Set aside.
In a bowl, whisk eggs together until well blended. Add the onions along with the remaining ingredients to the eggs.
Pour into a greased 9 x 13 inch dish. Bake at 350 degrees for 35-40 minutes until a knife inserted in the center comes out clean. Let stand 10 minutes before cutting.