Lightly grease a rectangle baking dish with oil or non-stick spray.
In a large skillet, heat the butter over medium heat until it melts and foams up.
Add the onion and garlic and cook for 5 minutes.
Add the sun-dried tomatoes and cook for about 5 more minutes. Turn off the heat.
In a separate bowl, beat the eggs with the mustard.
Whisk in the milk.
Stir in the s & p.
Fold in the cheese, frozen potatoes and the stuff in the skillet.
Pour all of this into the baking dish. (At this point the casserole can be baked immediately or covered and refrigerated for up to 24 hours)
Bake at 350º for 35 - 45 minutes, or until the eggs are firm and the top is slightly golden. Serve right away if possible, or keep warm for up to an hour, if needed.